Base: Combine chocolate, butter, granulated sugar, vanilla and egg; mix well. Add graham cracker crumbs, coconut and walnuts; mix well. Press into 9" square pan. Chill.
Filling: Combine butter, milk and custard powder. Blend in icing sugar. Spread over base; chill 15 minutes.
Glaze: Partially melt chocolate with butter over hot water. Remove from heat; stir until entirely melted. Spread over custard layer. Chill; store in refrigerator.