Bottom Layer: Melt the butter, sugar and cocoa in top of a double boiler. (Or if you promise to be gentle, you can do it on the stovetop in a regular pot over low heat.) Whisk in the egg and stir to cook and thicken. Remove from heat and stir in the crumbs, nuts and coconut. Press firmly into an ungreased 8"x8" or 9"x9" pan.
Middle Layer: Cream the butter, cream and custard powder in a large bowl with an electric mixer; gradually add the icing sugar and beat until smooth and spreadable, adding a little extra sugar or cream if needed to achieve a frostinglike consistency.
Top Layer: Melt chocolate and butter over low heat. Cool. Once cool, pour over middle layer, spread evenly, and chill. Cut into squares.