For the filling: Line a 13 by 9-inch pan with whole graham crackers. Melt the butter in saucepan and add sugar. Beat the egg and milk together; add to butter mixture. Bring to a boil, stirring constantly. Remove from heat. Add the nuts, coconut, and 1 cup graham cracker crumbs. Pour over the crackers in the pan. Cover with another layer of whole graham crackers.
For the topping: Beat all the ingredients together and spread over top layer of crackers.
Chill. Cut into squares.