Preheat oven to 350 degrees (or whatever temperature your box specifies). Line a 13- by 9-inch baking pan with foil, allowing several inches of excess to hang over the sides. Spray the foil with cooking spray and set pan aside.

Mix the brownies according to the directions on the box, substituting beer for the water. Fold in the crushed pretzels and spread batter into an even layer in the prepared pan. Bake according to the directions on the box and cool completely before frosting.

While brownies are in the oven, place the 1/2 cup of beer in a small saucepan. Set the pan over medium heat and bring to a boil. Reduce heat to low and simmer until the beer has reduced to about 1/4 cup. Set aside to cool completely before continuing with the frosting.

Once reduced beer has cooled to room temperature, place the butter in the bowl of a stand mixer fitted with the paddle attachment. Beat until very creamy, about 1 minute. Add the reduced beer, the remaining 2 tablespoons beer, vanilla, and salt and beat for 1 minute more. Add 3 cups of the powdered sugar and beat on low until combined, then increase speed to high and beat for 3 to 4 minutes, until very fluffy. If frosting is too thin, add additional powdered sugar. If it’s too thick, add additional beer (you don’t need to reduce it).

Spread frosting over the cooled brownies and top with sprinkles and/or pretzels. (I recommend refrigerating the brownies overnight to make slicing easier but it isn’t necessary.) Use the excess foil to remove brownies from their pan. Slice into bars and serve.