In a large bowl, or the bowl of your standing mixer, combine the softened butter and powdered sugar. Turn on to low, and mix until the butter and sugar begin to combine. Turn off the mixer.
Add in the cocoa powder, pinch of salt, vanilla, and 2 tablespoon of milk. Turn the mixer to low speed and mix until well combined.
Add additional milk as needed and and continue mixing the buttercream until you have reached your desired consistency.
Turn the mixer to medium-high and whip the buttercream until it is smooth and fluffy.
Notes
This buttercream can be used right away, stored in the refrigerator and brought to room temperature when ready to use, or even frozen for up to three months. Just thaw it out, and give it a quick mix in your mixer when you are ready to use it.