Preheat oven to 325 degrees.
Spray a 13" x 9" pan with cooking spray.
Prepare brownie mix according to package instructions.
Pour half of the mix (or a little less) and carefully spread to coat the bottom of the pan.
Unwrap your peppermint patties and line them up in the pan, 6 x 4.
Pour remaining batter over the patties, carefully, and spread so all candy is covered.
Place in the oven for about 18-20 minutes. Watch closely as they finish. You don't want the candy to bubble up out of the top of the brownies. Just trust me on that one. (Ovens do vary. You can check the center with a toothpick for doneness.)
Allow to cool, and refrigerate for several hours to allow the candy to set. Let the brownies come to room temperature before cutting and serving.