Layer 1: Heat margarine, white sugar, and cocoa in top of double boiler and stir till smooth. Add beaten egg and vanilla. Remove from heat. Add graham crumbs, coconut and walnuts. Spread mixture in pan and place in refrigerator to set. Layer 2: Cream butter, icing sugar, custard powder, and milk. Spread over the first mixture and return to refrigerator to set. Layer 3: Melt chocolate squares and butter in double boiler and spread hot and very thinly over the second layer, and refrigerate once more. Cut into small squares to serve!