Cream butter and sugar until combined. Add egg yolks and mix until combined (set whites aside for the icing.) Add the zest and the flour and mix until just combined, then add juice and mix until combined.
Scoop out heaping teaspoons of dough, then roll them into balls between your hands. Place on a cookie sheet and bake for 13 minutes. Remove from the oven and keep on the cookie sheet for 3 to 4 minutes. Remove from the pan with a spatula and allow to cool completely before icing.
To make the icing, combine 1 egg white with the rest of the icing ingredients. Whisk thoroughly until combined, adding either more powdered sugar or more juice until it reaches a pourable but still thick consistency.
Drizzle the icing across the cookies in several lines, then do it again in the other direction. Sprinkle with extra zest before the icing sets.