01 - Beat the softened butter with an electric mixer on medium speed until smooth and creamy, about 1 minute.
02 - Gradually add the powdered sugar, beating on low speed until fully incorporated. Scrape down the sides of the bowl as needed.
03 - Add the milk, vanilla extract, and salt. Beat on medium-high speed for 2–3 minutes until the frosting becomes light and fluffy.
04 - For a thinner consistency, add more milk 1 teaspoon at a time. For a firmer frosting, add additional powdered sugar.
05 - Spread or pipe the frosting onto completely cooled sugar cookies. Allow to set at room temperature for 1–2 hours to form a slight crust.