Preheat & Prep. To start, preheat the oven to 350 degrees F. Line two sheet pans with parchment paper and set them aside.
Blot The Pumpkin. Line a plate with paper towels and spread the pumpkin puree out onto the lined plate. Blot with additional paper towels to remove extra moisture from the puree, until you only have 1/3 cup pumpkin puree left. (You may need to use some extra paper towels!)
Whisk The Dry Ingredients. In a medium bowl, combine gluten-free flour, oats, baking soda, pumpkin spice, and salt until well blended, then set aside.
Cream Butter & Sugars. In the bowl of a stand mixer (or a large mixing bowl), cream the butter, brown sugar, and sugar 2-3 minutes with the paddle attachment (or a hand mixer), until smooth and creamy.
Finish The Wet Ingredients. Scrape down the sides of the bowl, then add pumpkin puree, egg yolk, and vanilla extract and mix until evenly blended. (It may look a tiny bit separated here--don't panic!)
Combine The Dough. Next, scrape down the sides of the bowl again and add the flour mixture. Mix until just blended, then add chocolate chips and mini chocolate chips and stir until evenly distributed.
Scoop & Bake. Use a medium cookie scoop to measure 2 Tablespoon size scoops of cookie dough onto a prepared baking sheet. Bake 6 cookies at a time, for 8-10 minutes. When the cookies come out of the oven, garnish with additional chocolate chips or mini chocolate chips, if desired. Then, transfer cookies to a wire rack to finish cooling. Repeat with the remaining cookie dough, alternating cookie sheets with each batch so you don't put cool dough on a hot cookie sheet.
Serve & Store. Enjoy the cooled cookies and store leftover cookies in an airtight container at room temperature 2-3 days or freeze in a freezer bag up to 2 months.