In a mixing bowl, combine the softened unsalted butter and powdered sugar. Mix on low speed until the sugar is fully incorporated.
Add the milk, vanilla extract, and a pinch of salt to the mixture. Beat on medium speed until the frosting is smooth and creamy, which should take about 2-3 minutes.
If you prefer a thicker frosting, you can add more powdered sugar, one tablespoon at a time, until you reach your desired consistency.
If you want to color your frosting, divide it into smaller bowls and add food coloring to each bowl until you achieve the desired shades.
Use a spatula to spread the frosting on cooled cookies or pipe it onto the cookies using piping bags for more decorative designs.
This frosting can be stored in an airtight container in the refrigerator for up to one week. Before using, let it sit at room temperature for about 10 minutes and re-whip if necessary.
Feel free to top your frosted cookies with sprinkles or edible glitter for a festive touch.