Dissolve hartshorn in milk and set aside. Beat eggs till thick and lemon-colored (10-20 minutes). Slowly beat in the powdered sugar, then the softened butter. Add the hartshorn and milk, salt, preferred flavoring, and grated rind of lemon or orange, if desired. Gradually beat in as much flour as you can with the mixer, then stir in the remainder of the 2 lbs. of flour to make stiff dough. Turn onto floured surface and knead in enough flour to make a good print without sticking.
It is not required, but I find the dough easier to work with when I refrigerate it for 8 hours before printing the cookies. (Refrigerating the dough is particularly helpful in humid conditions.) Seal the dough tightly in zippered plastic bags and in a sealed container so the dough does not dry out. The liquid ingredients absorb into the flour making it less sticky. You can refrigerate the dough for up to 3 days.
On a floured surface, roll dough into a flat pancake approximately 1/2 inch thick. Roll thinner or thicker based on the depth of the carving in the cookie press you are using. Shallow carvings will need to be thinner while deeper carvings will need to be thicker. Flour your cookie mold for each and every pressing. Press the mold firmly and straight down into the dough, then lift, cut and place the formed cookie onto a flat surface to dry. (I like to put the formed cookies directly onto a parchment lined cookie sheet, but you may put them on a countertop or tabletop covered with flour sack cloths if you do not have enough cookie sheets.)
Do not cover the cookies while they dry. The goal of drying is to set the design. Let the cookies dry at least 12 hours, but you will loose clearness of design. Best is a 24 hour drying period before baking to get a clean print after baking. Larger cookies and warm humid weather may require longer drying times. Cookies that are not dried long enough will not retain the beautiful designs, but will taste fine.
Bake on greased or baker’s parchment-lined cookie sheets at 255° to 325° till barely golden on the bottom, 10-15 minutes or more, depending on size of cookie. See Baking Times chart below for suggested baking times and temperatures. (I set my oven at 255.)
Store in airtight containers or in zipper bags in the freezer. Cookie stored in tins in a cool dry conditions are best in my opinion. Tins seal but allow a slight amount of air circulation. Springerle keep for months, and anise flavor Springerle improve with age. Orange and lemon and almond flavors have a fully develop flavor when they are baked and cooled. Yield 3 to 12 dozen. Yields vary greatly depending on the cookie presses you use.