BOTTOM LAYER: In double boiler, melt butter, sugar and cocoa; add egg and cook until thickened. Add crumbs, nuts and coconut. Press into ungreased 9 inch square pan. MIDDLE LAYER: Beat together butter, cream, custard powder and sugar; spread over base. Chill. TOP LAYER: Melt chocolate with butter; cool slightly. Pour over second layer; chill. Cut into bars.