Mix together all the base ingredients (butter, sugar, cocoa, egg, graham wafer crumbs, coconut, and walnuts). Press the mixture into a 9-inch square pan and refrigerate.

To make the filling, cream together butter, custard, and vanilla. Gradually blend in milk and icing sugar alternately. After filling is smooth, spread onto the base, and refrigerate well before putting the icing on.

To make the icing, melt chocolate and butter together. Spread on chilled mixture. Refrigerate until chocolate sets. If you let it sit too long in the fridge, the icing will "crack" when you try to slice it. Once the chocolate has set, cut into bars and store in fridge.

Note that you can use your microwave for all the ingredients that need to be melted.

A nice touch at Christmas is to substitute peppermint extract for vanilla in the filling, and tint it green with food colouring.