In a large bowl mix together butter, pumpkin, sugar, powdered sugar, vanilla, and eggs until smooth.
In another bowl whisk together flour, baking soda, cream of tartar, salt, and pumpkin pie spice.
Gradually mix dry ingredients into wet ingredients until incorporated. Wrap dough in plastic wrap and chill for 30 minutes.
Preheat oven to 350 and lightly grease a baking sheet with cooking spray.
Roll dough into 2-inch balls and place on baking sheet. Use the bottom of a tall glass (or a small jar) to flatten the dough balls to 1/2 inch thickness. Bake for 10-14 minutes until cookies start to look "dry". Allow to cool completely on the baking sheet.
To prepare the frosting, cream together butter and cream cheese in a medium bowl. Mix in sour cream. Gradually mix in powdered sugar. Store in airtight container.
Store cookies in an airtight container (separate layers of cookies with wax paper or parchment paper) Frost cookies immediately before serving.