Beat the butter on a medium speed for 30 seconds with a paddle attachment, until smooth. Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.
If the frosting is too thick, add in additional cream (1 teaspoon at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).
If you plan to color the buttercream, add in the gel food coloring once the frosting is fully made, and beat on low until it reach the desired colored
If you are making frosting for a cake, it is important to mix the buttercream on low at the end of the process for a couple minutes, to get out any extra air that might have be incorporated during the mixing process. This will make it easier to get super smooth sides on your cake!