Chocolate ganache filling
- Beat egg whites until stiff but not dry, adding granulated sugar.
- Next, mix in the egg yolks one at a time, just enough to combine.
- Sift together flour, baking powder, and cornstarch; gradually and gently fold into the egg mixture to combine - be careful not to deflate the egg whites by over-mixing.
- Line a large nonstick spring-form pan with parchment paper and fill with the batter.
- Bake in a preheated oven at 350 °F (175 °C or gas mark 2) for 30 minutes - it will only rise up partway.
- Let cool completely.
- Meanwhile, heat ½ cup of the cream in a small pan over medium-low heat, then stir in the milk chocolate until it melts and is smooth.
- Remove pan from heat and let cool for 1 hour at room temperature.
- Finely grate the bittersweet chocolate using a microplane grater and divide into two portions.
- Beat 1½ cups of the cream until stiff peaks form.
- Stir chopped nuts and half (one portion) of the grated chocolate gently into the cooled melted chocolate mixture.
- Gently fold together the whipped cream and the cooled melted chocolate mixture until just combined.
- Remove ring from spring-form pan and carefully slice the cake into three even (very thin) layers using a large serrated knife.
- Place one cake layer on a large plate, spread with ⅓ of the chocolate ganache, then top with another cake layer, another layer of ⅓ of the ganache, followed by another cake layer.
- Spread the remaining chocolate ganache mixture all over the top and sides of the cake.
- Chill for 2 hours.
- Knead the marzipan(with enough powdered sugar to make it sweet if unsweetened) until well blended.
- Place marzipan between two sheets of wax paper and roll out into very flat circle, about the size of your spring-form pan.
- Set a plate on top of the marzipan and use the plate as a guide to cut out a perfect circle; then cut circle into 16 triangles.
- Set the marzipan triangles in a spiral around the top of the chilled cake, pressing down slightly so they stick - there should be very narrow gaps between each triangle - if desired, curl up the pointy tips decoratively first.
- Coat the sides of the cake with some the remaining grated chocolate.
- Beat the remaining ¾ cups of cream with the vanilla sugar to taste until stiff peaks form.
- Shortly before serving, garnish cake with swirls of the whipped vanilla cream, the remaining chopped nuts, and sprinkle any remaining grated chocolate on top.