Melt the butter and let it cool down a little. Put the butter biscuits and shortbread in a freezer bag, seal it and crumble finely with a rolling pin. Mix crumbs and butter well and put them into a springform pan (26 cm Ø).
Press firmly with your hands to form a base, forming a rim about 1 cm high. Keep cold.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Roughly chop the chocolate. Melt 150 g chocolate in a hot water bath, then let it cool down a little.
Mix cream cheese, cocoa, sugar, vanilla sugar and 3 eggs with the whisks of the mixer. Stir in melted chocolate briefly. Put the mixture into the springform pan on the crumb base and smooth it down.
Bake the cake in a hot oven on the lowest shelf for about 45 minutes.
Remove from the oven and place on a cake rack. Increase oven temperature (electric oven: 200 °C/circulating air: 175 °C/gas: see manufacturer). Melt 50 g chocolate in a hot water bath. Spread evenly on the cake.
Sieve 150 g icing sugar and starch. Beat 3 egg whites, 1 pinch of salt, lemon juice and sugar mix with the whisks of the mixer in a hot water bath until stiff. Beat until the egg white mass binds a little.
Spread loosely on the cake, leaving an approx. 1 cm wide edge free.
Bake in a hot oven for 10-12 minutes. Remove from the oven, place on the cake rack and immediately remove from the edge of the springform pan all around with a knife. Leave to cool in the mould. Remove from the mould and dust the rim with 1 tsp. icing sugar.