Beat the butter with an electric mixer on high until light and fluffy, 3 to 5 minutes. Reduce mixer speed to low. Gradually add the sugar, then the peppermint extract and salt, and beat until smooth, scraping down the sides of the bowl as necessary.
Transfer ½ cup of the frosting to a small bowl and mix with the red food coloring.
Scoop the red frosting onto one side of a pastry bag (fitted with a ½ inch tip) or a plastic bag. Spread the frosting into a strip that runs the length of the bag. Scoop remaining white frosting into bag. Cut a ½ inch opening at the tip of the plastic bag, if using.
Pipe the frosting onto the cupcakes, moving in a circular motion to make swirled mounds.