Preparing the Meringue Layers:
- Toast and chop hazelnuts: Preheat the oven to 350°F (175°C). Spread the hazelnuts on a baking sheet and toast for about 10 minutes, until fragrant. Once cooled, rub off the skins and chop finely.
- Prepare meringue batter: In a clean, dry bowl, beat the aged egg whites with cream of tartar until soft peaks form. Gradually add sugar, beating until stiff peaks are achieved. Gently fold in the chopped hazelnuts and flour.
- Bake meringue layers: Draw two 9-inch circles on parchment paper and place on baking sheets. Spread the meringue mixture evenly within the circles. Bake at 250°F (120°C) for about 2 hours, or until dry and crisp. Turn off the oven and let the meringues cool inside.
Preparing the Sponge Cake Layer:
- Beat eggs and sugar: In a mixing bowl, beat the eggs and sugar until light and fluffy. Add vanilla extract.
- Incorporate dry ingredients: Sift together flour and baking powder. Gradually fold into the egg mixture until just combined.
- Bake the sponge cake: Pour the batter into a greased and floured 9-inch round cake pan. Bake at 350°F (175°C) for 20-25 minutes, or until a toothpick inserted comes out clean. Let it cool completely.
Preparing the Buttercream Frosting:
- Cook egg mixture: In a saucepan, whisk together eggs, sugar, and milk. Cook over medium heat, stirring constantly, until thickened. Remove from heat and cool to room temperature.
- Combine with butter: Beat the softened butter until fluffy. Gradually add the cooled egg mixture, beating until smooth. Add vanilla extract and rum or cognac if desired.
- Divide frosting: Split frosting into two portions. Add cocoa powder to one portion for chocolate buttercream.
Assembling the Cake:
- Layer the cake: Place one meringue layer on a serving plate. Spread a thin layer of apricot jam, then a layer of plain buttercream. Place the sponge cake layer on top, repeat the jam and buttercream layers, then top with the second meringue layer.
- Frost the cake: Use the remaining buttercream to frost the top and sides of the cake.
- Decorate: Drizzle with chocolate glaze or melted chocolate and garnish with chopped hazelnuts.
Chill: Refrigerate the cake for several hours or overnight to allow the flavors to meld and the meringue to soften slightly.