2. With an electric mixer, beat the butter until smooth, about 2 to 3 minutes.
3. Gradually add the confectioners' sugar and beat until the mixture is pale and fluffy.
4. Beat in the egg yolks, vanilla extract, salt and lemon zest.
5. Stir in the flour.
6. Chill the dough for about 15 minutes, until firm enough to handle, or 1 hour if you want to make cut-out cookies. Shape into 4 logs that are 1 1/2" in diameter and cover with plastic wrap. Chill. (This can be done in advance and stored in the refrigerator for up to 3 days or in the freezer for 1 month.)
7. When the logs of dough are very firm, slice them about 1/4"-thick and place 1" apart on an ungreased baking sheet. If making cut-out cookies, roll the dough on a floured surface and cut as desired.
8. Bake until very pale golden brown, about 8 to 10 minutes. Remove from the baking sheet and let cool.