In a large bowl, cream the softened butter, sugar and brown sugar.
Add in the egg, vanilla and pumpkin puree. Stir to combine all. Don't worry if it's not smooth.
In a separate bowl, combine the flour, baking soda, cream of tarter and salt.
Add the dry ingredients to the wet ingredients. Stir until flour is absorbed.
Cover and refrigerate the cookie dough for 30 - 60 minutes. If you're short on time, you can place the dough in the freezer for 10 - 15 minutes.
Combine the remaining 1/4 cup of sugar and cinnamon. Set aside to roll the cookies in prior to baking.
Preheat oven to 350 degrees.
Scoop the cookie dough into even sizes, approx. 1 - 2 tbsp. each. Roll into a ball and roll the ball completely in the sugar and cinnamon mix.
Place cookies on parchment paper or a silicone baking mat, 2 inches apart.
Bake for 12 - 14 minutes until done.
Remove from oven and allow cookies to cool before removing from the baking tray (they'll be a little delicate while they're hot!).