In a small bowl, combine the granulated sugar and the lemon zest. Using your fingers, rub the zest into the sugar until all of the sugar has been moistened. In the bowl of a food processor, combine the lemon sugar, all-purpose flour and salt. Pulse a few times to combine. Add the butter and continue to pulse until the pieces of butter are no larger than the size of peas, about 10 to 12 pulses.