Preheat oven to 350°F (175°C). Grease a 9×13-inch pan. Prepare cake batter according to box directions, spread into pan, and bake 25 to 30 minutes until a toothpick comes out with a few moist crumbs. Set on a wire rack.

In a saucepan over medium heat, whisk together evaporated milk, sugar, cocoa, butter, and salt. Bring to a simmer and cook 3 to 4 minutes, stirring, until smooth. Remove from heat and stir in vanilla.

Poke holes all over the warm cake with a skewer. Slowly pour the warm soak over the cake, letting it pool into the holes. Let sit 10 minutes, then cover and refrigerate at least 2 hours.

Spread marshmallow creme evenly over the chilled cake with an offset spatula.

Position an oven rack 4 to 5 inches from the broiler. Broil the cake for 1 to 2 minutes, watching closely, until the topping is golden and puffy. Slice and serve warm or at room temperature.