For the chocolate sauce, boil 60 ml water, 50 g sugar and 1 pinch of salt in a pot. Stir in the cocoa with a whisk. Bring everything to the boil while stirring constantly. Remove from the heat, stir in 50 g of cream and let it cool down for about 30 minutes.
For the mascarpone cream, mix mascarpone, vanilla sugar and 3 tablespoons of sugar. Beat 200 g cream until stiff, fold into the mascarpone in 2 portions. Bring 700 ml milk to the boil. Stir pudding powder, 6 tbsp. sugar and 100 ml milk until smooth. Stir into the boiling milk, bring to the boil and simmer for about 1 minute. Remove from the heat, let it cool down for about 15 minutes, stirring several times.
Line the bottom of a square springform pan (approx. 24 x 24 cm) with 12 biscuits. 1⁄3 Spread the pudding and half of the mascarpone cream on top. Sprinkle with 1⁄3 chocolate sauce. Layer 12 more biscuits, 1⁄3 pudding and the rest cream on top. 1⁄3 Drizzle gravy over it. Finish with 12 biscuits, remaining pudding and the rest of the chocolate sauce. Chill overnight.
Cut the chocolate bars and 4 biscuits into small pieces. Remove the cake from the tin and sprinkle with biscuit and chocolate bar pieces.