Switch to the paddle attachment (to avoid air bubbles) and add all the soft, cubed butter all at once. It's very important that the butter is soft and the same temperature as the meringue, otherwise you risk the mixture splitting. Leave the mixer on a medium speed until the mixture comes together (it may look split to start with, but don't worry it will come together), then turn down to low until you have a smooth buttercream. Add the cocoa powder and whip until incorporated.