Cook beer in a small saucepan over medium heat for 10 minutes or until reduced to ¼ cup.

Preheat oven to 350°. Spray a 9x9-inch baking pan with nonstick spray; set aside.

Place mix into large bowl. Add eggs, reduced Guinness, and vegetable oil. Combine with a spoon until combined and moistened. Spread batter into a greased 9×9-inch baking pan.

Bake for 38 to 40 minutes or until toothpick inserted 2 inches from side of pan comes out almost clean; cool completely.

For frosting: Stir together frosting and sundae syrup in a small bowl. Spread over cooled brownies; top with pretzels and drizzle with additional butterscotch sundae syrup, if desired.

Store covered at room temperature up to 5 days.