Place the flour, sugar, baking powder, zest, and salt 
     in a food processor and process for 20 seconds.  ( Or
     whisk them together by hand in a large mixing bowl.)
     Distribute the butter evenly over the flour and process 
     until the mixture resembles coarse meal, 15 to 20 
     seconds.  ( Or rub the butter into the flour with your 
     fingertips, or cut it in with a pastry blender.)  In a small 
     cup, whisk together the cold water and egg yolks.  With
     the processor running, pour the egg mixture in a steady
     stream through the feed tube and process just until the 
     dough comes together, 20 to 30 seconds.  ( Or sprinkle 
     the egg mixture over the flour mixture while tossing 
     with a fork.)  Place the dough on a lightly floured work 
     surface, and knead it several times to bring it together.
     Shape the dough into two thick disks, wrap each one in 
     plastic wrap, and refrigerate them for 1  hour.  Roll and
     shape cookies as in previous recipe.  Bake at 375*
     for 20 minutes, until crisp, firm to the touch, and golden 
     around the edges. 

     Makes about 3 dozen.