In a mixing bowl, sieve confectioner's sugar and cocoa powder, whisk well and set aside.
In the bowl of an electric stand mixer, fitted with a paddle attachment, place butter and beat on medium high speed until pale and creamy, scraping down the sides of the bowl, as required, about 2-3 minutes. You can also use a hand mixer.
Next, add the sugar-cocoa powder mixture, 1/4 cup at a time. Beat on lowest speed until all the sugar-cocoa powder mixture is well combined with the butter.
Add vanilla extract, and beat on medium low speed another few seconds.
Stream in the heavy cream, a tablespoon at a time, scraping down the sides and bottom of the bowl as required, until the frosting is smooth and fluffy.
Frost the cooled cakes/cupcakes using nozzles, or simply spread the frosting with an offset spatula! Yummmm !!