Mix 1/2 cup butter or margarine, white sugar, cocoa, egg and vanilla in a heavy sauce pan or double boiler. Stir over low heat until mixture is of custard like consistency.
Combine graham crackers, coconut and walnuts and add to the melted mixture. Mix well and pack into buttered 9 inch square cake pan.
Cream 1/4 cup butter, confectioners' sugar, vanilla custard powder and milk. Beat until creamy and spread over melted base.
Refrigerate till hardened.
Melt semi-sweet chocolate with 1 tablespoon butter and drizzle over custard icing. Refrigerate. When totally hard cut into square bars.
