Add the butter to a stand mixer with a paddle attachment, or large mixing bowl with a handheld mixer. Beat it for 1 to 2 minutes until smooth. 1 cup unsalted butter
Add in the powdered sugar, 1/2 cup at a time until each is mixed in completely. Scrape the bottom and sides of the bowl as needed. 3-4 cups powdered sugar
Add in the heavy cream (starting with 3 TBS), vanilla extract, and salt on medium speed and mix until everything is combined. 3-4 TBS heavy cream, 1 tsp vanilla extract, 1 dash salt
Add additional heavy cream, or powdered sugar as needed to get the right consistency for a spreadable frosting.
Mix the frosting for another 2 to 4 minutes at medium speed until it is smooth and fluffy.
Separate the frosting into 3 or 4 portions and add a few drops of food coloring to each if desired. Mix into the frosting until evenly combined. Food Coloring
Beat the frosting with a rubber spatula by hand to get air bubbles out.
Use the frosting to spread with a knife or pipe it onto your prepared cookies. (Also works on cakes, cupcakes, etc.)