Ingredients:
For the Cake:
Nonstick spray
1 1/2 cupsDutch-process cocoa powder, sifted, plus more for the pans
3 cupsunbleached all-purpose flour, plus more for the pans
3 cupsdemerara or granulated sugar
1 1/2 teaspoonsbaking powder
1 1/2 teaspoonsbaking soda
2 1/2 teaspoonskosher salt
4 large eggs plus 2 yolks, room temperature, beaten
1 1/2 cupslow-fat buttermilk, room temperature
1 1/2 cupshot tap water
4 teaspoonspure vanilla paste
3 tablespoonsmalted milk powder
3/4 cupvegetable oil
Buttercream frosting (below)
Milk chocolate and dark chocolate malted milk balls (I like the ones from Whole Foods), lightly crushed, for serving
For the Buttercream
2 large eggs plus 2 yolks, room temperature
1/2 teaspoonkosher salt
3/4 cupgranulated sugar
1 1/4 cupsunsalted butter, room temperature, cut into tablespoon sized pieces
7 ouncesdark (70%) chocolate, melted and cooled
2 teaspoonspure vanilla paste
3 tablespoonsmalted milk powder
Directions:
Heat the oven to 350°F. Generously spray two half sheet trays, line with parchment, and spray again; dust with cocoa powder, tapping out excess. In a large bowl, whisk together the flour, sugar, cocoa, baking powder, baking soda, and salt. Whisk in the eggs, buttermilk, water, vanilla, milk powder, and oil.
Divide the batter evenly between the prepared pans. Bake until the tops spring back when lightly pressed, about 30 minutes. Transfer the pans onto a wire rack; let cool 15 minutes. Invert the cakes, remove the parchment, then turn right side up (to prevent sticky tops from sticking to rack); let cool completely.
Spread the frosting over top of one cooled cake layer. Top with the remaining layer. Spread a thick coat of frosting over top. Sprinkle with crushed malted milk balls.
Do Ahead: Unfrosted cake layers can be stored, wrapped in plastic, at room temperature or refrigerated overnight. The frosted cake can be refrigerated overnight.
Beat the eggs and yolks and salt in a stand mixer fitted with a whisk attachment until pale yellow, thick, and ribbony, about 5 minutes.
Meanwhile, bring the sugar and 3 tablespoons water to a boil in a saucepan over high heat and cook until the mixture reaches 238°F; remove from the heat.
With the mixer on low speed, slowly pour the hot syrup so that it streams down the sides of the bowl, cooling slightly before hitting the egg mixture. When all of the sugar syrup has been added, increase to high speed and beat until light, fluffy, and cool, about 8 minutes.
Add butter a piece at a time, beating to incorporate completely before adding another piece and scraping the sides of the bowl often. Gradually add in the chocolate, vanilla, and milk powder, scraping the sides and bottom of the bowl occasionally, until completely incorporated and the frosting is thick, fluffy, and creamy.