In a large bowl (or the bowl of a stand mixer) cream together the butter, sugars, and lemon zest on medium speed. Add the egg yolks and lemon juice to the butter mixture and cream until smooth. Add the flour mixture and then mix until just combined.
Bake for 9-10 minutes or until the edges are firm and the tops have beautiful cracks. Place on a cooling rack and enjoy warm, room temperature, (or even cold!)