Using an electric hand mixer or stand mixer, cream the butter.
Add the salt and about a cup of the powdered sugar, and mix well to incorporate. Pour in about 1 tbs of the Big Red, then mix well to incorporate. Continue, alternating between sugar and Big Red until at least 3 cups of powdered sugar has been added and 1 tbs Big Red remains.
Taste to determine if the flavor and texture are to your liking. Add more Big Red as needed, but be aware that you'll likely need to add more powdered sugar along with it to keep a creamy texture.
Frost the cake using a piping bag, butter knife, or offset spatula.
If you want to make the Big Red flavor stronger in the icing, try reducing it into a syrup ahead of time. Simmer 3 cups Big Red over medium high heat for about 25 to 30 minutes. You should end up with about 1/2 cup Big Red syrup that you could use in place of Big Red for the icing. It'll need to cool completely before making the icing. Keep in mind, though, that because cooking it neutralizes the carbonation, this wouldn't be good to put in the cake itself.