Mix the crushed biscuits or cookies with the melted butter and milk until combined. Press the mixture firmly into an 18-20 cm (7-8 inch) pan. Refrigerate while you prepare the filling.
Beat the mascarpone, powdered sugar, and peanut butter until smooth. Add the heavy cream and beat until the mixture is thick and spreadable.
Spread the filling over the crust and refrigerate for 1 hour.
Heat the heavy cream until just boiling. Pour it over the chopped chocolate, let it sit for a few minutes, then stir until smooth.
Let the chocolate mixture cool to lukewarm, then pour it over the cake.
Garnish with chopped peanuts, if using, and refrigerate for 3 hours or overnight.
Slice and serve.