Preheat the oven to 180C. Combine the crushed biscuits with the butter and the flour and press into the base and sides of a 20cm cake dish. Freeze for 10 minutes, and then bake in the oven for 10 minutes to crisp off. Turn the oven off and return the dish to the fridge until needed.
For the peanut butter cream, beat the peanut butter, icing sugar and butter together until smooth. Pour over the crushed biscuits and pop back in the fridge.
For the chocolate mousse, soak the gelatine in cold water while you get on with everything else. Set a bowl over a pan of simmering water and melt the chocolate. When completely melted pop the bowl to one side to cool. Get another bowl on the saucepan but this time with the egg whites and sugar. Whisk constantly until doubled in size.
Mix the egg yolks into the chocolate, and then add the gelatine. Fold in the egg whites, followed by the cream. Spread this over the peanut butter goop and again return to the fridge.
For the cream topping, whip the cream and sugar together until stiff and top the cake with it. Shave or grate the remaining chocolate over the top. You could eat it straight away but it’s best to leave it for an hour in the fridge.