Whip your butter with the paddle attachment if you have one, or just a general beater with your mixer.
Add in your icing sugar, one cup at a time, beating continously. Now 2 tablespoons lemon zest worked wonderfully for me. Zested peel should not be done on a regular grater, by the way, you need a smaller size, a zester grater, in fact. The size is perfect without being overwhelming chunks in your cooking.
The 3 tablespoons of lemon juice was just perfect for my tastes, zingy and zippy with a sweet background of pure sugar and butter. I then added the 2 teaspoons of cream to thin it out.
This recipe will frost around 18 cupcakes, but that depends on how much icing you use on each cupcake! It will also frost one (( 2 layer) cake.