One-Bowl Chocolate Sheet Cake with Fudgy Frosting

This one-bowl chocolate sheet cake is the dessert I reach for when I need something that comes together fast and feeds a crowd without fuss. The batter gets mixed by hand in a single bowl, the crumb is tender and moist, and the warm poured frosting sets into a glossy fudgy layer.

Hot coffee deepens the chocolate notes, but you can swap in water if you prefer. The whole thing goes from mixing bowl to oven in about 15 minutes, and it’s the kind of cake that makes even a Tuesday feel a little celebratory.

Why This Cake Is So Good

The batter starts out thin and bakes into a cake with a tight, soft crumb that stays moist for days. I’ve made it the day before a party and it held up beautifully, the frosting staying just as silky as when it was fresh.

That frosting is really what makes it. You cook butter, cocoa, and milk on the stovetop for a minute, then beat in powdered sugar until it’s thick and shiny. Poured over the still-warm cake, it spreads easily and firms up into a topping that’s part fudge, part frosting, with a slight crackle on top.

Ingredients

• 2 cups (250 g) all-purpose flour: spooned and leveled, not scooped directly
• 2 cups (400 g) granulated sugar: regular white sugar keeps the texture light
• 3/4 cup (60 g) unsweetened cocoa powder: natural or Dutch-processed both work
• 2 tsp baking soda: gives the cake its lift
• 1 tsp fine sea salt: use slightly less if your butter is salted
• 1 cup (240 ml) buttermilk: at room temperature for smoother mixing
• 2 large eggs: also room temperature
• 1/2 cup (120 ml) vegetable oil: keeps the crumb tender without a mixer
• 2 tsp vanilla extract
• 1 cup (240 ml) hot brewed coffee: or hot water, coffee just amps up the chocolate flavor

For the frosting:

• 1/2 cup (113 g) unsalted butter: cut into pieces, salted butter works if you drop the pinch of salt
• 1/4 cup (20 g) unsweetened cocoa powder
• 1/3 cup (80 ml) whole milk: 2% is fine, don’t use skim or the frosting may be thin
• 3 cups (360 g) powdered sugar: sifted if it has lumps
• 1 tsp vanilla extract
• Pinch of fine sea salt

Variations / Substitutions

• Make it without coffee: replace the hot coffee with the same amount of hot water; the cake will still be deeply chocolatey.
• Use buttermilk substitute: stir 1 tablespoon lemon juice or white vinegar into 1 cup whole milk, let it sit 5 minutes, then measure what you need.
• Swap the frosting: a thick chocolate buttercream or cream cheese frosting works on a completely cooled cake instead of this warm poured version. The texture will be fluffier, less fudgy.
• Add chocolate chips: fold 1/2 cup mini chocolate chips into the batter before baking for little pockets of melted chocolate.
• Go darker: sub half the cocoa powder with dark cocoa for a more intense, almost espresso-like flavor.

If you want a smaller version of this cake, the recipe halves easily into an 8-inch square pan. Just keep the baking time close to 20 minutes and start checking early.

How to Make One-Bowl Chocolate Sheet Cake

Step 1: Mix the Batter

Preheat the oven to 350°F (180°C) and grease a 9×13-inch baking pan (metal works best for even browning). In a large bowl, whisk together the 2 cups flour, 2 cups sugar, 3/4 cup cocoa powder, 2 teaspoons baking soda, and 1 teaspoon salt until no lumps are visible. Add the 1 cup buttermilk, 2 eggs, 1/2 cup oil, and 2 teaspoons vanilla. Whisk just until combined, then pour in the 1 cup hot coffee. Stir until the batter is smooth.

The batter will be thin, closer to pancake batter than a thick cake batter. That’s exactly what you want—it’s the reason the cake bakes up moist and tender instead of dense. A few small lumps are fine; don’t overwork it now.

Step 2: Bake the Cake

Scrape the batter into the prepared pan and slide it into the oven. Bake for 25 to 30 minutes, until a toothpick poked into the center comes out clean or with a few moist crumbs clinging. The top should feel springy when you touch it lightly.

About 20 minutes in, the cake will start pulling away from the edges of the pan just slightly and the whole kitchen will smell richly of chocolate. Let it cool in the pan on a wire rack while you make the frosting—you want the cake still warm when the frosting goes on, so don’t let it sit too long.

Step 3: Simmer the Frosting

In a medium saucepan over medium heat, melt the 1/2 cup butter. Whisk in the 1/4 cup cocoa powder and 1/3 cup milk, stirring until the mixture is smooth and starts to bubble. Let it boil gently for 30 seconds, stirring constantly so it doesn’t scorch. Remove the pan from the heat and whisk in the 3 cups powdered sugar (sifted if it needs it), 1 teaspoon vanilla, and a pinch of salt. Beat until the frosting is thick, glossy, and completely smooth.

The frosting will be pourable right now but sets up quickly as it cools. Work fast from here—pour it over the warm cake immediately so it spreads evenly and starts firming into that fudgy layer almost at once. A large spatula helps you nudge it all the way to the corners.

Step 4: Frost and Slice the Cake

Pour the warm frosting all over the cake and spread it gently to the edges. Let it rest for about 20 minutes so the frosting can set, then cut into squares right in the pan. Serve the pieces while they’re still a little warm or let them come to room temperature.

The first cut reveals a tender crumb with a thick, soft chocolate top that has a faint sheen. The frosting stays creamy even after the cake cools completely, but that just-set, slightly gooey state is my favorite reason to grab a slice early.

A Few Tips for the Best Sheet Cake

• Room-temperature buttermilk and eggs: stir in easily and keep the batter from seizing. Set them on the counter an hour ahead or warm the buttermilk gently in the microwave (about 20 seconds).
• Don’t overmix once the wet ingredients go in.: Stir just until the flour disappears; overworking at that point can make the cake tough instead of tender.
• Sift the powdered sugar: only if it’s lumpy. Lumps don’t dissolve in the hot frosting mixture and will leave a grainy texture.
• If you only have salted butter, skip the pinch of salt in the frosting and reduce the salt in the cake to 3/4 teaspoon.

Bake times by pan size:

Keeping the Cake Fresh

• Refrigerate: cover the pan tightly and refrigerate for up to a week. The frosting stays fudgy even when cold.
• Serve cold or at room temp: the cake is good straight from the fridge, but if you have a few minutes, let slices sit out for 15 minutes to lose the chill.
• Reheating: warm individual slices in the microwave for 10 to 15 seconds if you want the frosting to soften again.

Frequently Asked Questions

Can I make this cake gluten-free?

Yes, a good 1:1 gluten-free flour blend (the kind with xanthan gum) works in place of the all-purpose flour. The texture will be slightly more tender, but still moist and sturdy enough to hold the frosting.

Why did my frosting turn out grainy?

A grainy frosting usually means the powdered sugar wasn’t fully dissolved or was added with lumps. Whisk the hot butter mixture thoroughly after adding the sugar, and sift the powdered sugar beforehand if it has any clumps.

Can I freeze the cake?

Absolutely. Freeze the unfrosted cake, well wrapped, for up to 2 months. You can also freeze it already frosted; just wrap tightly and thaw overnight in the fridge. The frosting will look the same once it comes to room temperature.

How do I make a smaller version?

Halve all the ingredients and bake in an 8×8-inch pan. Start checking for doneness at 20 minutes. The frosting recipe also halves cleanly.

One-Bowl Chocolate Sheet Cake Recipe

Print Recipe Pin Recipe

Prep Time 15 minutes mins

Cook Time 25 minutes mins

Total Time 40 minutes mins

Servings: 12 servings

Calories: 520

Ingredients Method Notes

Ingredients

  

• 2 cups (250 g) all-purpose flour
• 2 cups (400 g) granulated sugar
• 3/4 cup (60 g) unsweetened cocoa powder
• 2 tsp baking soda
• 1 tsp fine sea salt
• 1 cup (240 ml) buttermilk, at room temperature
• 2 large eggs, at room temperature
• 1/2 cup (120 ml) vegetable oil
• 2 tsp vanilla extract
• 1 cup (240 ml) hot brewed coffee or hot water
• 1/2 cup (113 g) unsalted butter
• 1/4 cup (20 g) unsweetened cocoa powder
• 1/3 cup (80 ml) whole milk
• 3 cups (360 g) powdered sugar
• 1 tsp vanilla extract
• Pinch of fine sea salt

Method

 

1. Heat oven to 350°F (180°C). Grease a 9×13-inch pan. Whisk flour, sugar, cocoa, baking soda, and salt in a large bowl. Add buttermilk, eggs, oil, and vanilla; whisk until just combined, then stir in hot coffee until smooth.
2. Pour batter into pan. Bake 25-30 minutes until a toothpick comes out clean. Cool in the pan on a rack while you make the frosting.
3. Melt butter in a saucepan over medium heat. Whisk in cocoa and milk, bring to a boil, and boil 30 seconds. Remove from heat and whisk in powdered sugar, vanilla, and salt until thick and glossy.
4. Pour frosting immediately over the warm cake, spread evenly, and let set about 20 minutes. Slice and serve warm or at room temperature.

Notes

For the deepest chocolate taste, use the hot coffee; water works well too. If you don’t have buttermilk, stir 1 tablespoon lemon juice into 1 cup milk, let it curdle 5 minutes, and measure what you need. The cake keeps beautifully covered at room temp for up to 3 days.