Recipe video above. The key to ensuring your buttercream comes out fluffy and smooth (rather than dense and gritty) is to beat, beat, beat and to use the right ratio of butter to icing sugar. I like to make my chocolate buttercream with Dutch Processed cocoa powder, but you can use chocolate if you prefer, refer to the notes.
Place butter in the bowl of a stand mixer with paddle attached. Beat for 3 minutes on speed 7 until it becomes pale.
Add half the icing sugar and cocoa powder. Beat for 3 minutes - start slow, increase to speed 7.
Add remaining icing sugar and cocoa powder. Beat again for 3 minutes.
Add milk and vanilla if using. Beat again for 1 minute on speed 7.
Spread onto cakes / cupcakes, or spoon into frosting bag and pipe.
1. Icing Sugar - if you are in Australia, get the packets labelled as Soft icing sugar. Not Pure Icing sugar- that's for royal icing, the stuff that goes hard that's used for writing. 2. Quantity - This makes enough to swirl frosting on 12 cupcakes (see photo in post) or frost one 22cm / 9" cake (top, sides and filling) a reasonable amount.To frost cake more generously, like as depicted in this Chocolate Cake recipe (2 layer 22cm/9" cake), increase the recipe by 50%. Use the recipe scaler (hover /click on servings and slide) and increase until icing sugar becomes 450g / 3.75 cups.3. Dutch Processed cocoa powder is a more intense flavour than normal cocoa powder. So you will get a better chocolate flavour.4. To make this using chocolate instead of cocoa powder, use 75g / 3 oz 70%+ cocoa dark chocolate. Melt and cool, then whisk in at the end.