In a bowl, beat the butter, custard powder, and vanilla. Gradually add the icing sugar and milk (The mixture should be thick but spreadable). Spread over the base. Refrigerate until firm, about 1 hour.
In a bowl, stir together the graham crumbs, coconut, walnuts, cocoa, and sugar. Stir in the butter and egg. Mix until combined. Press into parchment lined 9 inch square pan. Bake in the preheated to 180 degrees C (350 degrees F) oven for about 10 minutes, until firm. Let cool.