Line an 18cm round cake tin with a sheet of baking paper. For the mousse cake, just blitz together the mascarpone, icing sugar, eggs, 200g melted dark chocolate and cocoa powder to a smooth cream. Break 2/3 of the biscuits roughly and fold into the mixture. Spoon into the prepared tin and even out. Bake in the oven at 150`c for 40-45 minutes. Allow to cool down. To serve, spoon over the melted white chocolate and crumble the remaining biscuits and on top.