Add the butter to a mixing bowl. Using a hand mixer, or a stand mixer with the paddle attachment, cream the butter on medium-low speed.
Add the vanilla, cocoa powder, and salt. Mix on low speed.
Add 1 cup of the powdered sugar and mix, starting on low speed.
Add 1 Tablespoon of the milk and mix in, then add another cup of the powdered sugar and mix it in.
Add more milk if needed, just 1 Tablespoon at a time, until the frosting is at a spreadable consistency.
Taste and add more powdered sugar to get to the sweetness you like. Then add a little more milk, if needed.
Mix on medium-high speed to make the frosting fluffier.
Use right away, or store in the fridge in an airtight container with plastic wrap covering the frosting, touching it, so it doesn't get a "skin" on it.
Let chilled frosting come up to room temperature and re-whip it before spreading or piping.