Preheat oven to 350F. Combine butter and sugars in a medium bowl, do not over mix. Stir in pumpkin puree until smooth. Add egg and vanilla, stir until combined.
In a separate bowl mix together flour, cream of tartar, baking soda, pumpkin pie spice and salt.
Fold dry ingredients with the wet ingredients. The mixture will be somewhat moist, but that's how it should be.
In a small bowl combine the ingredients for the spiced sugar coating. Line a baking sheet with parchment paper.
Spoon up a heaping tablespoon of dough. Gently roll dough into balls using a light touch and just the palms of your hand. I found the dough is slightly sticky on the finger tips, but if you roll it in the palms of your hand gently you can get a nice, soft dough ball. The melted butter in the recipe should help it keep it from sticking to your hands. Dough balls were approximately 1 1/2in to 2in in diameter. You may need to adjust baking time if you make them smaller.
Place the dough ball in the spiced sugar coating and gently toss to coat. I found the dough is so soft that while it will hold shape, it will start to lose that shape the more you handle it and try to roll it around. The more sugar coating you can toss onto it instead of rolling it, the better. Gently remove the sugared dough ball from the coating and carefully place it on the baking sheet using your fingertips to fix any major misshaping of the ball.
Bake cookies for 10 minutes. Remove from the oven and allow to cool for a few minutes before transferring to a cooling rack.