To make the pastry, place the flour on a work surface, make a well, and add the butter, the egg yolks, the sugar, the salt, and the lemon zest. Using your finger tips, mix these ingredients until they are thoroughly combined, then incorporate the flour by bringing it into the center with your fingers and gently mixing it with the wet ingredients. When you have crumbly but cohesive dough, form it into a round and let it sit, covered, at room temperature for 30 minutes.