Preheat oven to 350°F and line a large baking sheet with parchment paper.
In a medium mixing bowl, sift together the oat flour, tapioca flour, sweet rice flour, baking soda, salt, and spices. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugars together starting on low speed and increasing to medium-high speed, until well combined, 1-2 minutes. Scrape down the sides and add the egg, pumpkin, molasses, and vanilla extract and beat until just combined.
With the mixer on low speed, slowly add the dry ingredients until just combined. Scoop 1½ tablespoon-sized balls and place them 2½ inches apart on the parchment-lined baking sheet (I use the OXO medium cookie scoop and it makes exactly 21 cookies). Chill for 10 minutes in the fridge, or 5 minutes in the freezer.
Meanwhile, prepare the cinnamon sugar for rolling. Mix the ¼ cup of granulated sugar with 1 teaspoon ground cinnamon. Roll each cookie dough into a ball in the palm of your hand, then roll in the sugar to coat. Place dough balls 2-3 inches apart on the baking sheet. For thinner cookies, move to step 6 and bake immediately. For thicker cookies, chill the dough before baking (15 minutes in the freezer or 30 minutes in the fridge).
Bake in batches, if needed, for 11-12 minutes, until the edges are just set, but the centers look underbaked. Let cool for two minutes on the baking sheet before transferring to a cooling rack with a thin metal turner to cool completely. Sprinkle with flaky salt, if desired.
Notes
Flour Substitutions:
Gluten-Free Oat Flour: If you don't have oat flour, you can make your own by grinding gluten-free rolled oats in a blender or food processor until fine, then sifting out any larger pieces. Alternatively, you can sub in teff flour by weight for a darker cookie with a slight toffee flavor, or use Bob's Red Mill 1-to-1 gluten-free flour. by weight. For a grain-free version, see below.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.