Place softened, unsalted butter into mixing bowl. Whisk on high speed for about 5 minutes, until butter is light and fluffy. Scrape down sides of mixing bowl often to get all the butter mixed well.
Add the dulce de leche (cooked sweetened condensed milk) or caramel sauce. If necessary, increase the amount of confectioner’s sugar to thicken the frosting. Mix for about a minute until the caramel is well combined with the butter.
Add the vanilla and salt. Start by adding 1/4 teaspoon salt, then add more if needed. Begin adding the confectioner's sugar 1 cup at a time, scraping down sides of mixing bowl often to get everything incorporated. Make sure to sift confectioner's sugar before adding to avoid any clumps. After adding all sugar, whisk on high speed for 2 to 3 minutes to get frosting very fluffy.