Chocolate-Coffee Dacquoise

1 . Stage

Preheat the oven to 250 degrees F (120 degrees C).

2 . Stage

Mark out a 13x10-inch rectangle on parchment paper; place it on a baking sheet.

3 . Stage

Combine ground almonds, ground hazelnuts, 1/2 cup of caster sugar, cornstarch, and salt in a bowl.

4 . Stage

Whip egg whites and cream of tartar together in another bowl until foamy, then mix in the remaining 1/2 cup caster sugar until glossy and stiff. Fold in the nut-sugar mixture in two batches. Spread the meringue onto the parchment paper and mist the surface with water.

5 . Stage

Bake in the preheated oven for 90 minutes, then turn off the oven and leave the meringue inside for another 90 minutes. Remove to a wire rack to cool.

6 . Stage

Whisk egg yolks, confectioners' sugar, cornstarch, and salt for buttercream together in a bowl.

7 . Stage

Heat milk to a simmer. Pour milk into the yolk mixture, whisking until smooth. Return mixture to the heat and continue whisking until thickened and stiff. Transfer custard to a bowl, cover, and refrigerate until set, about 2 hours. Before using, bring to room temperature, about 30 minutes.

8 . Stage

Dissolve instant coffee in hot water.

9 . Stage

Whip butter in a bowl until light. Beat in the custard in 3 batches until just combined. Add coffee mixture and beat until light and fluffy.

10 . Stage

Place chocolate for ganache in a heat-proof bowl. Heat cream and corn syrup to a simmer in a small saucepan; then pour over the chocolate to soften. Stir until smooth and cool for 5 minutes.

11 . Stage

Peel off paper from meringue and transfer to a cutting board. Use a serrated knife to trim the edges and cut into 4 equal rectangles. Spread 3 rectangles each with about 1/4 cup ganache. Refrigerate until firm, about 1 hour.

12 . Stage

Spread remaining rectangle with about 1/2 cup buttercream; place on a wire rack. Place one ganache-coated meringue, chocolate-side down on top and press gently. Spread on 1/2 cup buttercream and repeat with the remaining meringue. Frost the top and sides of the cake with the remaining buttercream. Refrigerate until firm, about 1 hour more.

13 . Stage

Microwave remaining ganache in a microwave-safe bowl in 15-second intervals, until warm and runny. Pour over cake and garnish with whole hazelnuts.