Place the biscuits into a bag and use a rolling pin to crush them finely. You can use a food processor if you have one.
Combine with the cocoa powder, vanilla extract, the melted butter and mix everything well.
Transfer the mixture to a round cake tin (mine is a 16X9cm), and use a a spoon to press the mixture down firmly into an even layer.
Refrigerate while you get on with the cream.
Melt the milk chocolate over a pan of simmering water, then add the mascarpone and use a silicone spatula to combine everything gently.
Remove the tin from the fridge, spread the chocolate mixture over the biscuit base, then refrigerate again for at least an hour.
Meantime, pour the double cream into a pan and add the dark chocolate to it, broken into pieces.
Set the pan over a low heat and stir continuously until the chocolate melts, making sure you don’t boil it.
Remove from the heat and leave to cool.
Spread it over the milk chocolate layer, and refrigerate again overnight, if not, for at least 4 hours.
Once set, top it with grated white chocolate.