Samosa Dough (julie Sahni)

Mix the flour with the baking soda and salt; make a well in the center and put the shortening in the well. Pick up some flour and fat in one hand. Slide the other hand lightly over this mixture, moving from heel to fingertips, letting the fat-coated flour fall back into the bowl. Pick up more fat and flour and continue this sliding action until the flour is evenly coated; it should have a fine texture with no lumps. Add the yogurt-water mixture a little at a time. Add just enough liquid until the dough come together in a mass. Depending on the flour and the humidity, you may need more or less liquid. Knead the dough until it's smooth and elastic. The dough is ready when it's no longer sticky and feels as soft as an earlobe, after about 5 minutes of kneading. Roll the dough into an 8-inch log, wrap in plastic, and let rest for about 15 minutes. (It can also be wrapped tightly and chilled for a day or frozen for longer--about 5 days. Bring it to room temperature before continuing. If frozen, thaw in the refrigerator before bringing it to room temperature.) Cut the dough into eight even pieces. Roll each piece into a ball and flatten slightly. Keep the pieces you're not working with covered with plastic wrap. On a floured surface, roll a flattened piece of dough into a thin 6-inch round. To get even results, roll from the center of the dough to the edge and turn the dough frequently. Rolling back and forth will toughen the dough. Cut the circle in half--one half makes a samosa. Moisten half of the straight edge with a little flour-water mixture. Create a cone by bringing the dry half of the edge over the moistened half. Overlap them by 1/4 inch. Press the seam together to close. Be sure to pinch closed the point of the cone as well. Fill the cone with two heaping tablespoons of the potato mixture and seal the open side by continuing to pinch the straight edge to slightly thin and extend it.