Prepare brownie mix according to package directions and cool completely. Cut into 1 inch squares.
In a large bowl combine pudding mix with 2 1/2 c. milk. Mix until smooth, then fold in 8 ounces whipped topping until no streaks remain.
In a trifle bowl or glass serving dish, place half of the brownies, half of the pudding mixture and half of the 12 ounce container of whipped topping. Repeat layers. Shave chocolate onto top layer for garnish. Refrigerate 8 hours before serving.