Slowly beat the butter and sugar, in another medium bowl, with an electric mixer until incorporated. Raise the speed to high and mix until light and fluffy, about 5 to 7 minutes. (Occasionally turn the mixer off, and scrape the sides of the bowl down with a rubber spatula.) Add the fluff and salt and reduce the speed to low. Add the milk and vanilla mixture, scrape the bowl down, and mix until fully incorporated. Raise the mixer to high and beat briefly until fluffy, about 1 to 2 minutes. Frost cupcake immediately.