Combine butter and cocoa in a large bowl. Beat with electric mixer at low speed until blended, then on medium speed for another 2 minutes, or until smooth and creamy.
Add salt, powdered sugar, vanilla, and 3-4 tablespoons of the milk. Beat for another 2-3 minutes at medium-high speed until fluffy, scraping the sides of the bowl a few times.
Add more milk as needed to reach a desired consistency. Use a piping bag and tip, or spread on your cupcakes or favorite cakes.
You will need 3-4 tablespoons of milk for a thick pipeable frosting, 4-5 tablespoons for a spreadable frosting, and 5-7 tablespoons for a frosting that will run down the sides of a bundt cake.
Notes
The secret to fluffy frosting is beating it for a few minutes to incorporate air. Use caution with a stand mixer though, because if you beat it too long, it can split and appear curdled.
You can substitute evaporated milk, cream, or even buttermilk for the milk.
This will make enough frosting for 15-20 cupcakes, depending on how much you add to each cupcake.
Frosting can be stored in the refrigerator in an airtight container for at least a week. Before using, let come to room temperature, then beat until fluffy.