Chop 400 g chocolate. Boil up the cream. Pull the pot off the stove. Melt the chocolate in the hot cream while stirring. Pour into a bowl and cover directly with foil. Leave to cool, then chill for at least 4 hours.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Line a baking tray with baking paper. Finely grate 50 g chocolate. Separate the eggs. Beat the egg whites and 1 pinch of salt until stiff, adding 125 g sugar and vanilla sugar at the end. Mix flour, cocoa, espresso powder and baking powder, sieve on top. Carefully fold in grated chocolate and milk. Spread the dough evenly on the tray. Bake in a hot oven for about 10 minutes. Turn the sponge cake onto a tea towel sprinkled with sugar. Peel off baking paper. Let the sponge cake cool down.
Cut the sponge cake in half crosswise. Stir the jam until smooth and spread on one half of the sponge cake. Briefly whip 500 g of the chocolate cream with the whisks of the mixer. Spread the cream on the base and smooth it down. 2. place the cake base on top. Cover with baking paper and weigh down with a board, chill for approx. 2 hours.
Chop 100 g chocolate and melt in a hot water bath. Cut the sponge cake plate into cubes (approx. 4 x 4 cm) with a sharp knife, dipping the blade repeatedly in hot water. Spread the chocolate on the cubes.
Briefly whip the remaining chocolate cream with the whisks of the mixer and fill into a piping bag with perforated spout. Spray dab onto each cube. Sprinkle with chocolate sprinkles and chill until serving.