Mix and boil the butter and margarine for 3 minutes. Arrange graham crackers, broken into squares, on a cookie sheet (make sure the cookie sheet has a raised edge, or this could get messy!) Pour cooked mixture over crackers. Sprinkle chopped pecans over butter and cracker mixture. Bake at 350 for 8 minutes. Remove from oven. Let stand 1 minute. Remove to waxed paper. Sprinkle with powdered sugar.