Prepare brownie batter according to the package directions. Bake in a 9x13 baking pan @ 350* for 20-25 minutes. Cool on a wire rack; cut into 3/4" pieces. Cut peanut butter cups in half; set aside 1/3 Cup for garnish. In a large bowl, whisk milk and pudding mixes for 2 minutes, mixture will be thick. Add the peanut butter and vanilla, mix well. Fold in 1 1/2 of the tubs of Cool Whip. Place 1/3 of the brownies in a 5 qt. Triffle Bowl; top with 1/3 of the remaining peanut butter cups, spoon 1/3 of the pudding mixture over the top. Repeat layers twice. Cover with remaining whipped topping and garnish with peanut butter cups. Refrigerate until chilled well.