For Chocolate Cake:
1⁄2 cup plus 2 tablespoons (12⁄3 ounces/45 grams) unsweetened cocoa powder, preferably Dutch-processed
3 tablespoons all-purpose flour
2 tablespoons cornstarch
5 large eggs, separated
2⁄3 cup (43⁄8 ounces/125 grams) sugar
4 large egg yolks
1 teaspoon salt
1⁄4 cup (13⁄4 ounces/50 grams) canola, vegetable, or other neutral oil
3 tablespoons buttermilk

For Malted Chocolate Mousse:
3⁄4 cup (6 ounces/168 grams) whole milk
1⁄2 cup (31⁄2 ounces/98 grams) sugar
1⁄4 teaspoon salt
1⁄4 cup (11⁄3 ounces/37 grams) Horlicks powder
6 large egg yolks
2 gelatin sheets, soaked in cold water until softened and drained, or 11⁄4 teaspoons powdered gelatin, softened in
1 tablespoon cold water
5 ounces (150 grams) bittersweet chocolate, finely chopped
1 cup (8 ounces/226 grams)
heavy whipping cream

For Vietnamese Coffee Buttercream:
1⁄2 cup (4 ounces/113 grams) unsalted butter, at room temperature
1⁄2 cup (2 ounces/55 grams) confectioners’ sugar
1⁄4 teaspoon salt
1⁄4 cup plus 1 tablespoon (11⁄2 shots) Vietnamese drip coffee or espresso
2 tablespoons sweetened condensed milk
1⁄2 teaspoon vanilla extract

Method:
1. To make the chocolate cake: Preheat the oven to 350°F. Spray an 11 × 17-inch rimmed baking sheet with nonstick cooking spray, line with parchment paper, and spray the paper. Set aside.

2. Sift the cocoa, flour, and cornstarch together and set aside.

3. Put the egg whites in the bowl of an electric mixer fitted with the whisk attachment and whisk at medium-high speed until frothy. With the machine running, slowly add 1⁄3 cup of the sugar and continue whisking until medium-soft peaks form. (When you lift the whisk from the mixture, a peak will form and the top of the peak will fall back down.) Transfer the whipped whites to a large bowl.

4. Put the yolks in the electric mixer bowl and whisk on medium speed until the yolks break. With the machine running, add the salt and the remaining 1⁄3 cup sugar in a slow, steady stream. Continue whisking until the mixture is pale yellow, thick, and doubled in volume, about 12 minutes.

5. Whisk half the whites into the yolk mixture until fully incorporated, then gently fold in the dry ingredients until no traces of flour or cocoa remain. Add the remaining whites to the mixture and fold gently until only a few white streaks remain. Whisk the oil and buttermilk together until smooth and homogenous, then fold into the batter until completely incorporated.

6. Spread the batter evenly in the baking sheet. Bake for 5 minutes, then rotate the pan and bake until a tester comes out clean, about 5 more minutes. Cool completely in the pan on a rack.

7. Meanwhile, make the mousse: Put the milk, sugar, and salt in a small saucepan, stir well, and set over medium heat. Warm just until bubbles form around the edges, then whisk in the Horlicks until completely dissolved. Remove from the heat.

8. Put the yolks in a large mixing bowl and whisk until the yolks break. Add the warmed Horlicks mixture in a steady stream while whisking constantly. Return the mixture to the saucepan and cook over low heat, stirring constantly, for 2 minutes. Remove from the heat and add the softened gelatin. Stir until the gelatin melts, then stir in the chocolate until it melts completely. Let the mixture cool to room temperature, at least 2 hours.

Notes and Tips:
The cake is beautiful as is, but you can further decorate it by sifting Ovaltine or cocoa powder over the entire surface of the cake or by pressing Walnut Cookie (page 104) crumbs onto its sides.

Malted Chocolate Layer Cake Recipe