Soften butter by either melting it and cooling it until slightly hardened or taking it out of the fridge the night before.
Add butter to a large bowl and mix with milk, vanilla and salt. Next add cocoa powder by sifting it and whip with a whisk or electric mixer together until smooth. Next stir in powdered sugar by sifting it into the mixture. Scrape sides and whip again until light, fluffy and smooth, 1-2 minutes.
This recipe frosts one 9 or 8 inch cake or 24 cupcakes. In order to frost a double layered 9 inch cake, simply make two batches of this recipe, or double the recipe!
Notes
Store airtight at room temperature for up to 3 days, or store airtight in the refrigerator for up to 7 days, or store airtight in the freezer for up to a month.