Begin by fitting a stand mixer with the whip attachment or using a hand mixer and a large bowl. Whip the softened butter on high speed for about 10 minutes until it becomes very pale, light, and fluffy, almost tripling in size. Periodically scrape down the sides of the bowl with a rubber spatula to ensure even whipping.
Once the butter is adequately whipped, gradually add the sweetened condensed milk in three or four increments, fully incorporating each addition before adding the next. Continue to scrape down the bowl with a spatula as needed to maintain an even mixture.
After incorporating the sweetened condensed milk, check the frosting’s consistency and flavor. Adjust the amount of sweetened condensed milk as desired, but be careful not to add the entire can if the flavor and consistency meet your preferences earlier.
If the frosting appears separated or curdled at any point, chill it in the refrigerator for 15-20 minutes, then re-whip to achieve a smooth, cohesive texture.
Stir in the optional vanilla extract and salt to finish the frosting.
Use the prepared frosting to decorate your favorite cakes, cupcakes, or other desserts as desired.