In a stand mixer using the whisk attachment, beat the softened butter on high speed for at least a full 5 minutes, until pale in color and fluffy. If using a paddle attachment or handheld electric mixer, add 1-2 more minutes to the beating time. Scrape down the sides of the bowl using a rubber spatula as necessary.
Lower the mixer speed to medium and beat the sweetened condensed milk into the whipped butter in three stages, beating well between each addition.
Beat in the vanilla extract and continue to beat the frosting until it's smooth and creamy. (If you are adding salt to this recipe, add it now.)
If the frosting starts to curdle or separate, place the entire bowl (including the whisk attachment) into the refrigerator for 5-10 minutes and beat again.
Use immediately for frosting or decorating cakes.
Notes
If using unsalted butter, add 1/4 tsp. of salt to this recipe.