Prepare brownie mix batter according to package directions, using 1/4 cup oil instead of 1/2 cup as package directs. Stir peanut butter into batter. Spread half of brownie batter into an ungreased 13" x 9" x 2" baking pan. Place whole candy bars across batter. Spread remaining batter over candy bars.
Bake at 350° for 29 minutes. Cool completely on a wire rack. Cut into squares.