Preheat oven to 350 degrees. (Convection Bake mode works best) In a large mixing bowl or stand mixer, cream together butter, Crisco, sugar, powdered sugar and milk. Add eggs and mix until well combined. Add flour, baking soda, cream of tartar and salt, mixing well. Roll the dough into 1-inch balls and place on a cookie sheet lined with parchment paper or a Silpat. If you don’t have parchment paper or Silpat, grease the baking sheet. (See note below for difference size cookies) Spray the bottom of a glass or measuring cup with cooking spray (or butter) and then dip into a plate coated with a thin layer of sugar. Press each ball down slightly to get the rough edge that Swig cookies are known for. Cookies should be thick and soft, so do not smash down too much.) Bake for 8-9 minutes at 350 degrees. Cool on wire racks and then frost. To make the frosting, mix all of the ingredients except for the milk until well combined. Add milk to desired consistency and then frost the cooled cookies. To keep the cookies soft, store immediately in an airtight container.