Heat oven to 350°F. Grease bottom only of 13×9-inch pan with cooking spray. Make brownie batter as directed on box. Spread in pan.
Make cookie mix as directed on pouch. Drop dough by rounded tablespoonfuls evenly onto brownie batter; press down lightly.
Bake 35 minutes or until toothpick inserted 2 inches from side of pan comes out clean. Cool on cooling rack 30 minutes. Spread the frosting on top.