01 - Place softened butter in a large mixing bowl. Beat 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 to ensure even mixing.
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 frosting, add additional milk 1 teaspoon at a time. For firmer texture, incorporate more powdered sugar gradually.
05 - Spread or pipe frosting onto completely cooled sugar cookies. Let stand at room temperature for 1 to 2 hours to develop a slight crust.