Chocolate Pinwheel Cookies (prinzen Taler)

Mix shortening, sugar and egg together. Stir in milk and vanilla. Sift remaining dry ingredients together and stir into mixture. Divide dough into equal parts. Add chocolate and blend until incorporated uniformly. Chill dough. Roll out white dough into a 9 by 12 inch rectangle. Roll out the chocolate dough the same size, and lay over the white dough. Roll the double layer of dough gently until 3/16 of an inch thick. Roll up tightly, beginning at the wide end, into a roll 12 inches long and 2 inches in diameter. Chill dough. Slice 1/8 of an inch thick. Place slices a bit apart on a lightly greased cookie sheet. Bake 10 to 12 minutes in a 350 degree oven.