Cream butter, sugar and lemon zest in the bowl of an electric mixer. Finely chop basil and add to creamed mixture. Add egg yolks; beat in well. Add the flour and salt. Mix until just incorporated. Put dough between sheets of plastic wrap and flatten into a 1" thick disc. Refrigerate overnight. (Dough may be frozen at this point.)
Roll out dough on a floured board to about 1/8" thickness. Cut into 2" rounds with a cookie cutter and place on parchment paper lined cookie sheets. Bake ina 350°F oven 10-12 minutes, or until lightly browned. Transfer to racks to cool.
