Preheat oven to 350 degrees, place rack in center of the oven.
For topping, mix 1/4 cup sugar and 2 teaspoons cinnamon together in a small bowl. Set aside.
In the bowl of a stand mixer or large bowl, place the butter, dark brown and granulated sugar. Beat until smooth.
Add the pumpkin, egg, one egg yolk, and vanilla, beat on medium speed, scraping down the sides and bottom of the bowl one time during mixing.
Add all of the dry ingredients to the bowl at one time, mix on low until the wet and dry ingredients are incorporated. Or fold together using a spatula.
Using a cookie scoop or a spoon, form 2 inch balls of dough and roll in cinnamon sugar mixture. Place on a lightly greased cookie sheet or on a parchment lined cookie sheet. Barely flatten the cookie dough ball with the palm of your hand.
Bake at 350 degrees for 10-12 minutes or until the top is set and the cookie does not have any wet appearance on top. Remove from oven and let cool.
Notes
I HIGHLY recommend Libby's Pumpkin. Other brands will alter the final result of this recipe. Pumpkin varies from brand to brand, including color of pumpkin and the moisture content in the pumpkin.
If the cookie spreads too much while baking, use a small spatula and push the sides of the cookie back into a circular shape immediately after removing from oven.
If you do not have cream of tartar, you may use 1 1/2 teaspoons baking powder as a substitute. I used this substitution while living (and baking) in Spain, where cream of tartar is not available, and it works in a pinch!
The dough sets up well in the fridge or freezer. If freezing, I re-roll the cookie in the sugar mixture before baking.
If you would like to cut some of the butter in the recipe, I have had success with reducing the butter by 4 tablespoons. The cookie will need to bake for about 12-14 minutes.
I usually bang the cookie sheet on the oven rack or on the counter top at the end of baking time to remove any extra air in the dough, which also produces a more chewy texture in the cookie.
Chewy Snickerdoodles freeze well after baking. Let cool completely, place in an air tight container for 4-6 weeks.