Prepare the brownie mix according to the directions on the package.
Line a 13 x 9-inch cake pan with aluminum foil. Spray the bottom with cooking spray. Spread half of the brownie mix into the cake pan. Place the candy bars as close as possible to one another on the batter. You may have to break up one of the bars. Cover with remaining batter.
Since you are doubling up on the cake batter, these take about 35 - 40 minutes to bake. You can test their doneness by inserting a toothpick in the center. When it comes out clean, your brownies are done.
When the browinies are cool, the aluminum foil allows you to easily lift the brownies out of the pan and makes cutting them a breeze.