HEAT oven to 350 degrees F. Coat a 13 x 9-inch baking pan with no-stick cooking spray. Combine brownie mix, oil, water and eggs in large bowl; stir 50 strokes with spoon. Stir in 1 cup chopped pretzels. Spread evenly in prepared pan. Bake 28 to 31 minutes or until toothpick inserted in center comes out clean. Cool completely on wire rack.
STIR peanuts, caramel topping and remaining 1 1/2 cups pretzels in small bowl just until coated. Spread evenly over cooled brownies.
PLACE frosting into corner of 1-quart heavy-duty resealable plastic bag. Microwave on HIGH 5 seconds. Cut small corner off bag. Starting in one corner of pan, drizzle evenly in a left to right sweeping motion, ending at opposite corner of pan. Chill 5 minutes or until frosting is set. Serve immediately.