Mix together the icing sugar, lemon zest and butter until it becomes sort of crumbly.
Remove the zest with a sharp vegetable peeler in long strips, avoiding as much of the white pith as possible.
Previous versions of this recipe have included twice the amount of frosting. In this recent update I've halved that to provide enough frosting for the middle and top as many people seem to prefer. If you want to frost the entire sides of the cake as well. please feel free to make one and a half or even two times the recipe. It depends on how much frosting is your personal preference.