- Preheat the oven to 325 °F. Grease and line an 8-inch square pan with parchment paper so that the paper comes up the sides.
- To prepare the base, stir the cookie crumbs, sugar, cocoa and coconut together. Stir in the melted butter, and then stir in the egg and vanilla. Add the peanuts and press the mixture into the bottom of the prepared pan. The crust will be soft, so you can use dampened fingers to press it in evenly. Bake the crust for 15 minutes then cool while preparing the filling.
- Beat the butter with the icing sugar and custard powder until it forms a thick paste. Add the peanut butter, cream and vanilla and beat until smooth. Spread this over the cooled crust and chill until firm, about 2 hours.
- For the topping, melt the chocolate and butter in a bowl placed over a bowl of barely simmering water, stirring until melted. Spread this over the top of the chilled peanut butter filling and chill again until set, about an hour, before slicing into bars.
- Bars will keep refrigerated for up to 6 days.