Place the unsalted butter in a large mixing bowl. Whip the butter with stand mixer fitted with a whisk on medium-high speed for about 5 to 7 minutes. It needs to become pale and fluffy. You are incorporating air in the buttercream which makes it light and fluffy.

Add the vanilla extract and salt to the whipped butter, turning the mixer to low speed.

Slowly pour in the sweetened condensed milk, adding about a quarter of the can at a time. Mix on medium speed after each addition until well incorporated. Don't forget to use a rubber spatula to scrape down the sides and bottom of the bowl periodically.

If the frosting appears to be too soft or if it's not holding together well, you can add powdered sugar, a half cup at a time.