Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. Cut cookie dough in half crosswise; cut each section in half lengthwise. Press dough in bottom of pan.
Make brownie batter as directed on box for 13x9-inch pan, using water, vegetable oil and eggs; spread batter over cookie dough.
Bake as directed on brownie box; cool. For bars, cut into 6 rows by 4 rows; dust with powdered sugar before serving.