Combine the butter, salt, and confectioner's sugar in a mixing bowl fitted with the paddle attachment. Stir on low until thick and combined. (Trust me. You do not want a sugar explosion by putting the mixer on high too quickly.)
Add the milk and vanilla. Slowly increase the speed to medium-high and beat until fluffy and smooth. Scrape the bowl to ensure all the ingredients are combined fully. Beat another minute more.
Spread or pipe onto cakes or cupcakes as desired. PRO TIP: To store buttercream frosting, place plastic wrap directly on the surface to ensure no air can form a crust on top. This buttercream will keep at room temperature for 24 hours, and up to a week in the fridge. To softened refrigerated buttercream, let stand overnight at room temperature, or microwave on the defrost setting at 30-second increments. Rewhip until smooth.
Disclaimer: Nutrition information is provided for courtesy purposes only, and is an estimate not verified by medical or nutrition experts. Read the full nutrition disclaimer.