In the bowl of a mixer, cream together butter, sour cream and sugar. Beat in eggs. Add vanilla. In a bowl, combine flour, baking soda, salt and cream of tartar. Carefully add dry ingredients to batter. Beat until combined.
Using a cookie scoop, scoop out the dough, about the size of a walnut and with floured hands roll into a ball. Place on cookie sheet. Flatten cookies with the bottoms of a glass that has been dipped into sugar.
Bake for 10 minutes. Do not over bake. Let cookies rest on baking sheet for 2 minutes before removing to a cooling rack.
For the frosting, cream together butter and shortening. Beat in pumpkin. Add powdered sugar one cup at a time. Blend in pumpkin pie spice and a pinch of salt. If frosting is too thick add milk or water until creamy. Frost over completely cooled cookies.