Sift together the dry ingredients, except the sugars, three times. Set aside.
In a mixing bowl, blend together the butter and both sugars for 5 minutes- or until very light and fluffy. Make sure to scrape you bowl down at least twice during this stage.
Add the egg and mix until light and airy, about 4 minutes. Scrape down your bowl once again.
Stir in the pumpkin puree and vanilla and mix well. The mixture may look slightly curdled, it's all good. Scrape down the bowl once again.
Add the dry ingredients in three equal batches, mixing until each addition is absorbed before adding the next. Scrape down your bowl one final time and refrigerate your dough (in the bowl is fine) for at least 30 minutes.
Preheat your oven to 350°F. Line a sheet pan with parchment paper or lightly grease it.
Prepare the cinnamon-sugar: In a small bowl, combine the sugar and the cinnamon.
Remove the cookie dough from the fridge and scoop out a tbsp. Drop the cookie dough into the cinnamon-sugar and roll it to form a ball.
Place the ball on the sheet pan.
Continue scooping, dusting and placing until you have 1 dozen balls of snickerdoodle dough on your sheet pan.
Bake for 15-17 minutes.
Remove from the oven and, using a spatula, transfer the cookies to the cooling rack.