Adjust the oven racks to the upper-and lower-middle positions and heat the oven to 400 degrees F. Line two baking sheets with parchment paper; set aside.
In a large bowl, whisk together flour, cream of tartar, baking soda, salt and pumpkin pie spice; set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream the butter and 1 1/2 cups sugar at medium speed until light and fluffy, about 2 minutes. Scrape down the sides of the bowl with a rubber spatula as needed. Add the egg and beat at medium speed until combined, about 30 seconds. Add the pumpkin puree and beat at medium speed until combined, about 30 seconds. Add the dry ingredients and beat at low speed until just combined, about 30 seconds, scraping down the bowl as needed.
Place the remaining 1/4 cup sugar and cinnamon for rolling in a shallow bowl. Roll a heaping tablespoon of dough into a 1½-inch ball, roll the ball in the sugar mixture, and place it on the prepared baking sheet. Repeat with the remaining dough, spacing the balls about 2 inches apart
Bake the cookies, one sheet at a time, until the edges are set and just beginning to brown but the centers are still soft and puffy, 10 to 12 minutes, rotating the baking sheet halfway through baking (I realized that they can cook faster on the bottoms so I pulled my temp down to 375 and cooked closer to 10min). Cool the cookies on the baking sheets about 5 minutes; using a wide metal spatula, transfer the cookies to a wire rack and cool to room temperature.
I took a tin of them to a friend who I went to pilates with this morning – she was a happy camper (:
Cheers to fall at all times of the year