Beat butter with a hand or a stand mixer until smooth, then add the cocoa and mix until smooth. Add the powdered sugar 1 cup at a time, mixing in between each addition. Scrape down the sides of the bowl.
Add the salt and vanilla, mix until smooth.
Add 2 tablespoons of heavy whipping cream and beat on medium high speed until the frosting is smooth and fluffy and the desired consistency.
Note on mixing: if you’re using a hand mixer it’ll stay crumbly for quite some time; add the heavy whipping cream to help it come together.
If you feel like you need a little more to get to the consistency you like, you can add up to another 1 tablespoon, adding a teaspoon at a time. This differs each time you make it depending on what mixer you’re using or how hot it is (the buttercream comes together easier in summer, for example, when the kitchen is warmer).
Frost as desired; will frost one 9x13-inch cake or a 9-inch layer cake or 24 cupcakes. If you like extra frosting for decorating, make a double batch.
Store in refrigerator for up to 3 days and allow to come to room temperature before using; you may need to add some extra cream to thin it.
You can freeze the frosting too for up to a month. Just let it defrost in the refrigerator then at room temperature before using.
In place of heavy whipping cream you can use whipping cream, half and half, milk, or nonfat milk but the less fat in the milk the thinner the frosting will be, so use less.