I usually combine 2 packages of brownie mix and bake in a 13×9 pan. I used the Betty Crocker fudge brownie mix for the bake sale. Just follow the brownie package instructions, put the batter in the pan, and then follow this recipe for the peanut butter mixture:
Peanut Butter Swirl Brownies
1/3 cup packed brown sugar
1/3 cup crunchy peanut butter
In small bowl, mix brown sugar, peanut butter and egg. Drop by tablespoonfuls onto brownie batter. Cut through batter several times with knife for marbled design. Bake brownies as package instructions call for.