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