Ingredients
1 stick unsalted butter, melted and cooled, plus more, softened, for pan
1 1/3 cups unbleached all-purpose flour
1/2 cup Dutch-process cocoa powder, sifted
2 teaspoons baking powder
1 teaspoon kosher salt (we use Diamond Crystal)
4 large eggs, room temperature
1 1/2 cups granulated sugar
2 teaspoons pure vanilla paste or extract
1 cup sour cream, room temperature
2 sticks unsalted butter, softened
2 cups confectioners' sugar, sifted
1/2 cup Dutch-process cocoa powder, sifted
1/4 cup whole milk, room temperature
2 tablespoons chocolate sprinkles, for decorating
Preparation
Cake: Preheat oven to 350°F. Brush a 9-by-13-inch or 8-by-12-inch baking pan with butter. Line with parchment, leaving a 2-inch overhang on long sides. Butter parchment.
Whisk together flour, cocoa, baking powder, and salt. Combine eggs and granulated sugar in the bowl of a mixer fitted with the whisk attachment. Beat on high speed until thick and pale, about 4 minutes. Add flour mixture; beat just to combine. Add butter and vanilla; beat to combine. Add sour cream; beat until smooth.
Scrape batter into prepared pan, smoothing top with a spatula. Bake until top springs back when lightly touched and a tester inserted in center comes out clean, 30 to 35 minutes (for 9-by-13-inch) or about 40 minutes (for 8-by-12).
Let cool in pan on a wire rack 10 minutes. Using parchment overhangs, lift out of pan and let cool completely on rack. (Cake can be made to this point and stored at room temperature, wrapped in plastic, up to 1 day.)
Frosting: Beat together butter, confectioners’ sugar, cocoa, and milk on medium-high speed until light and fluffy, about 5 minutes. Spread over top of cake. Decorate with sprinkles. Frosted cake is best the day it’s assembled, but can be refrigerated, wrapped in plastic, up to 1 day. Return to room temperature before serving.
