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. Quantity - This makes enough to swirl frosting on 12 cupcakes (see photos in post), to smear on 18 cupcakes, or a single layer 20-22cm / 8-9" cake (like this Chocolate Fudge Cake).For a 2 layer cake, 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 3.75 cups (ie 3 3/4 cups) / 450g.