Increase the mixer speed to high and beat until the icing is completely smooth, glossy, and slightly thickened, 4 to 5 minutes. The icing should hold a soft peak and slowly run off of the whisk attachment. If the icing is too loose, add more powdered sugar, 1/4 cup at a time, and continue mixing until combined. Reduce the mixer speed to low and beat in 1 teaspoon vanilla extract.