Beat the butter and cocoa powder with an electric mixer in a large mixing bowl at high speed until fluffy, about 2-3 minutes.

Add half of the amount of sugar and mix on high for about 2-3 minutes. Then, add the remaining sugar and mix on high speed for another 2-3 minutes.

Add heavy cream (for creamy buttercream) or milk (less creamy) and mix again on high speed for 2-3 minutes.

Add the vanilla extract and mix again at high speed for 1-2 minutes. Add another tbsp of heavy cream or milk if the buttercream is too thick and beat again until smooth. If it's too thin, you can add more powdered sugar.