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).