Preheat the oven to 350°F.
Add the milk to the ammonium carbonate and let stand for 30 minutes, stirring frequently. Cream the shortening and the sugar; add the egg, lemon flavoring and the milk mixture. Stir in the flour. Chill. Roll the dough to one-fourth-inch-thick on a floured board. Cut into 3-inch squares. Prick with a floured fork. Bake on a greased baking sheets at 350 degrees for 15 minutes or until slightly browned.
Note: Ammonium carbonate is no longer readily available in stores. It can be special-ordered by many druggists, and sells for about $1.75 an ounce. Commercial bakeries use ammonium carbonate as a dry rising agent for various products, and may be willing to sell small batches to the public on request.
Makes three and one-half dozen.