CHOCOLATE-COFFEE DACQUOISE

Steps:

• Preheat the oven to 250 degrees F (120 degrees C).
• Mark out a 13x10-inch rectangle on parchment paper; place it on a baking sheet.
• Combine ground almonds, ground hazelnuts, 1/2 cup of caster sugar, cornstarch, and salt in a bowl.
• 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.
• 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.
• Whisk egg yolks, confectioners' sugar, cornstarch, and salt for buttercream together in a bowl.
• 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.
• Dissolve instant coffee in hot water.
• 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.
• 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.
• 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.
• 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.
• Microwave remaining ganache in a microwave-safe bowl in 15-second intervals, until warm and runny. Pour over cake and garnish with whole hazelnuts.