In a large mixing bowl, add the softened unsalted butter. Using your stand mixer or hand mixer, beat the butter on medium speed until it’s creamy and smooth. This should take about 2-3 minutes.
Spoon in the marshmallow fluff or crème into the bowl with the creamed butter. Mix on medium speed until the two are well combined, about 1-2 minutes.
Gradually add in the confectioners' sugar, one cup at a time. Mix on low speed to prevent a sugar cloud from forming. Once incorporated, increase the speed to medium and beat until fluffy.
Pour in the vanilla extract and add the salt. Mix until everything is well combined and the frosting is light and airy. If the frosting is too thick, you can add a tablespoon of milk to reach your desired consistency.
Give your frosting a taste and adjust the salt if necessary. If you prefer a sweeter frosting, feel free to add a bit more confectioners' sugar.
