Heat the oven to 350 degrees F. Coat a 10 x 15-inch rimmed baking sheet with nonstick cooking spray. Line with wax paper, and coat the wax paper with nonstick cooking spray.

Spoon brownie batter onto the prepared baking sheet. Bake for 18 to 20 minutes, or until a wooden pick inserted in the center comes out clean; let cool.

Remove the brownie from the baking sheet and carefully remove the wax paper. Cut into 2-inch squares.

Line a 2 1/2-quart bowl with plastic wrap and place about half the brownie squares into the bowl, pressing them together to cover the inside of the bowl. Spoon the vanilla ice cream into the bowl, and press it firmly over the brownies. Top with half of the remaining brownie squares, then spoon the raspberry sherbet over the top. Cover with the remaining brownie squares. Cover with plastic wrap and freeze for 2 to 3 hours, until frozen solid.

Remove the brownie bowl from the freezer and invert onto a serving platter, removing the bowl and plastic wrap. Frost with the whipped topping. Return to the freezer for 1 to 2 hours, then cover and keep frozen until ready to serve. Remove from the freezer 10 minutes before serving.