Sift the black cocoa and powdered sugar into a large bowl and set aside.
In the bowl of your stand mixer fitted with the paddle attachment, beat the butter on high speed for 5 minutes, or until pale and increased in volume.
Once the butter has become fluffy and has increased in volume, lower the speed to low and add the heavy cream, vanilla and salt.
With the mixer still running at low speed, begin to add the cocoa mixture, 1 cup (135 grams) at a time, until all has been added.
Increase the speed to medium and mix until creamy and well incorporate, but no longer than 1 to 2 minutes, or excess air bubbles will form in your buttercream.
If needed, add the black gel coloring at this point. Allow your mixer to run at the lowest setting for about 5 minutes to eliminate any excess air bubbles.
Remove any remaining bubbles by pressing the buttercream against the side of the bowl. You should hear popping sounds as the bubbles come to the surface and burst.
Continue to scrape and fold until the buttercream is very smooth and no visible bubbles remain. This can take quite a while sometimes, depending on how much air was incorporated into the buttercream when whipping. Take your time, and don't be afraid to take a breather if your arm is falling off from beating!
This buttercream can be stored for up to 1 week in the refrigerator, or 1 month in the freezer.