In a large mixing bowl, beat the butter until light and pale in color, about 2-3 minutes.
Add the powdered sugar, one cup at a time, and beat on low until all the powdered sugar has been added. Increase mixer speed to medium and beat until fully combined.
Add the vanilla extract and salt.
Add 1 tablespoon of milk and continue to beat, adding a little more milk until desired consistency is reached. Beat for an additional 2-3 minutes until the frosting is light and fluffy.
If coloring frosting, add the gel food coloring one drop at a time until desired color is achieved. Add the frosting to a piping bag.
After the cookies have completely cooled, pipe the frosting into the middle of the cookie cup. Top with 3 chocolate eggs.
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