Prepare the brownie mix according to package directions.
Line a 13 by 9-inch cake pan with aluminum foil and spray with vegetable oil cooking spray. Spoon in half of the brownie batter and smooth with a spatula or the back of a spoon. Place the candy bars side by side on top of the batter. Cover with the remaining batter.
Bake according to package directions. Let cool completely, then lift from the pan using the edges of the foil. This makes it easy to cut the brownies into squares.