Cream butter and cream cheese together - Place the cream cheese and butter in the bowl of a stand mixer fitted with the paddle attachment. Alternatively, you can use a hand mixer. Mix on low speed to begin with, to help break up the cream cheese a little, then increase the speed to high for a couple of minutes to help combine the cream cheese and butter.
Add sugar and milk powder - After a couple of minutes, stop your mixer and scrape down the side and bottom of the bowl with a spatula. Add the powdered sugar, milk powder and vanilla. Mix on low speed until the dry ingredients are fully incorporated, then bring the speed back up to high. Continue beating on high speed, about 5 minutes, until your frosting is fluffy and turns pale in color. Scrape down the bowl and beat on high speed for 2 minutes. Your frosting will be fluffy, pale in color and stiff enough to pipe.
Simply add the cocoa powder into the frosting along with the powdered sugar and beat as instructed. Add the melted and cooled chocolate to the frosting when you stop to scrape down the bowl, before the final 2 minutes of beating. Beat for 2 minutes, then stop the mixer and scrape down. Beat for a final 2 minutes.
