Melt first 3 ingredients in top of double boiler or in heavy saucepan. Add egg and stir to cook and thicken. Remove from heat. Stir in graham cracker crumbs, coconut and nuts. Press firmly in an ungreased 9-inch square pan.
Cream together the butter, milk, custard powder and confectioners' sugar. Beat until light. Spread over bottom layer.
Melt the chocolate and butter over low heat. Cool. When cool but still runny, spread over second layer. Chill in refrigerator. Use a sharp knife to cut.
Makes approximately 16 bars