In a medium bowl, combine powdered sugar, cocoa powder, and sea salt. Set this mixture aside for later use.
In the bowl of a standing mixer fitted with the paddle attachment, or in a large bowl with a hand-held mixer, cream the butter until it is light and fluffy. This should take a few minutes.
Add half of the powdered sugar mixture to the creamed butter. Slowly beat it into the butter until it's well incorporated. Then, turn the speed to high and beat until light and fluffy, about 30 seconds.
Add milk, vanilla extract, and the rest of the powdered sugar mixture. Beat on low speed to incorporate all ingredients, then increase to high and beat until smooth.
Scrape down the bowl and beat once more until the frosting is uniform in texture. Your frosting is now ready to use!
Notes
Use room temperature butter for better creaming, sift cocoa powder to avoid lumps, and adjust consistency with milk or more powdered sugar as needed. Consider adding instant coffee for added flavor.