Position a rack in the center of the oven and preheat to 350°F.
Starting on low speed and gradually increasing to medium, cream the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. Add the eggs, one at a time, paddling until absorbed into the mixture. Add the zest, and then add the water in a thin stream, letting it run down the side of the bowl.
Stop the motor and lower the bowl. Put the pastry flour, all-purpose flour, salt, and baking powder in a sifter and sift into the bowl. Starting on low speed and gradually increasing to medium, mix until the dough comes together.