For the Cake
- 1/2 cup (1 stick) plus 1 tablespoon unsalted butter, room temperature, plus more for pan
- 2 1/4 cups all-purpose flour, plus more for pan
- 2 1/4 teaspoons baking powder
- 3/4 teaspoon baking soda
- 1/2 teaspoon salt
- 1 cup plus 2 tablespoons sugar
- 3 large eggs
- 1 cup buttermilk
- 1 1/8 teaspoons vanilla extract
For the Frosting
- 3 tablespoons unsweetened cocoa powder
- 1 3/4 cups plus 2 tablespoons malted milk powder
- 1/4 cup plus 2 tablespoons boiling water
- 3/4 cup (1 1/2 sticks) plus 1 1/2 tablespoons unsalted butter, room temperature
- 10 1/2 ounces milk chocolate, melted and cooled
- Pinch of salt
- Chopped malted milk balls (optional), for decorating
- Make the cake: Preheat oven to 350 degrees. Lightly butter a 9-by-13-inch baking pan. Dust with flour, tapping out excess. In a large bowl, whisk together flour, baking powder, baking soda, and salt.
- With an electric mixer on high speed, beat butter and sugar until pale and fluffy, about 5 minutes. Beat in eggs, 1 at a time, until combined. Reduce mixer speed to low. Add flour mixture in 3 batches, alternating with 2 batches of buttermilk; beat to combine, scraping down sides of bowl as needed. Beat in vanilla.
- Transfer batter to prepared pan; smooth top with an offset spatula. Bake, rotating half way through, until cake is golden and puffed and a cake tester comes out clean, about 30 minutes. Transfer pan to a wire rack to cool completely.
- Meanwhile, make the frosting: In a medium bowl, whisk together cocoa and malted milk powders. Add the boiling water, whisk until smooth, and let cool. With mixer on high speed, beat butter until smooth. Add cocoa mixture; beat until combined. Add chocolate and salt. Raise mixer speed to medium- high; beat until frosting is smooth and peaks form, about 3 minutes. Spread frosting over cake; sprinkle with malted milk balls, if desired.
A completely cool cake is essential so that the frosting doesn't melt on contact. You can bake the cake a day ahead to ensure a fluffy frosting layer.