Preheat oven to 350°F. Line a 9×13-inch baking pan with parchment paper, or grease with cooking spray.
Prepare brownie batter according to package instructions.
Pour about 1/3 of the brownie batter into the prepared baking pan. Spread until the bottom of the pan is evenly coated. Then add two even layers of pretzels, covering the entire surface. Carefully spoon the remaining brownie batter on top to cover the pretzels.
Bake for 30 minutes, or until a toothpick inserted in the middle of the brownies comes out clean. Remove and set on a cooling rack to cool.