1 Preheat the oven to 180°C (approximately 350°F). Line the bottom of the springform pan with parchment paper.
2 For the cake, separate the eggs. Whip the egg yolks with the sugar, vanilla sugar and lemon zest until fluffy. Whip the egg whites until stiff and fold into the egg yolk mixture.
Mix the flour with baking powder and sift over the nuts. Combine the flour mixture with the egg mixture. Fill the springform pan and bake until golden brown and a toothpick inserted into the center of the cake comes out clean, about 35 minutes. Remove from oven, allow to cool briefly, unmold onto a wire rack and cool completely
Slice in half horizontally.
3 For the cream, whip the cream. Soften the gelatine in cold water. Mix the mascarpone, sugar and liqueur. Warm the gelatin and coffee in a pot and stir until dissolved. Add 2-3 tablespoons of whipped cream with the gelatin, then fold the coffee mixture into the whipped cream. Evenly spread about 2/3 of whipped cream on the base cake and cover with the cake top. Evenly spread the remaining whipped cream on the cake top and refrigerate for at least 2 hours.
4 For garnish, chop the dark chocolate and melt over a simmering water bath. Let cool slightly and drizzle over the cake. Garnish with chocolate curls and walnuts before serving.