Preheat oven to 400°F (200°C). Line two 8-inch round cake pans with parchment. Whisk flour, sugar, baking powder, baking soda, and salt. Cut in cold butter until mixture resembles coarse crumbs with pea-sized bits.

Whisk buttermilk, egg, and vanilla, then pour into dry ingredients. Stir just until no dry streaks remain, about 30 seconds. Dough will be wet and shaggy.

Divide batter evenly between pans, smooth tops slightly. Bake until golden and a toothpick inserted in the center comes out clean, 18 to 22 minutes. Cool 5 minutes in pans, then turn out onto a rack to cool completely.

While cake bakes, toss sliced strawberries with 2 tbsp sugar. Let sit at room temperature at least 15 minutes, stirring once or twice, until juicy and glossy.

In a chilled bowl, beat cold heavy cream, powdered sugar, and vanilla to medium-stiff peaks, about 3 to 4 minutes.

Place one cooled cake layer on a plate. Top with about a third of the whipped cream, then half the strawberries and some syrup. Add second layer, press gently, then top with remaining cream and berries. Serve immediately.