WET INGREDIENTS: In a bowl attached to a stand mixer (fitted with whisk attachment) add room temperature (not softened/melted) butter, brown sugar and white sugar. Beat, scraping edges as needed, until nice and creamy (peanut butter consistency). Add in the pumpkin, egg yolk, and vanilla. Mix to incorporate.
DRY INGREDIENTS: Add the cinnamon, pumpkin pie spice, baking soda, baking powder, salt, and quick oats to the mixture. Beat until combined. Add in flour and beat until no streaks of flour remain, scraping sides of the bowl as needed. Add in both chocolate chips and mix through.
CHILL DOUGH: Cover and place the bowl of dough in the fridge for 1 hour. (Don't skip this step!)
FORM COOKIE DOUGH BALLS: Remove dough from fridge and roll into large balls (3 tablespoons (58 grams) EACH. (See Note 4; batter should make 18-20 cookies.) Dough is a bit sticky and messy! Roll cookie dough balls to be taller instead of wider. Place rolled cookie dough balls on a parchment-paper lined tray or plate and return to the fridge for 15-20 minutes.
OVEN PREP: While the dough balls are chilling, preheat the oven to 350 degrees F. Line a large sheet pan with parchment paper or a silicone liner.
BAKE: Add only 6 cookie dough balls to a sheet pan at a time spacing them out well (they spread a lot). Bake for 12-15 minutes or until very lightly browned at the bottom/sides, but slightly gooey in the center. The key to these cookies is slightly under-baking - that will keep them soft and chewy in the inside. (Right out of the oven, hold both sides of the cookie sheet and bang the pan on the counter a few times to flatten!)
OPTIONAL: Working quickly, (right after banging the pan), use the back of a large spoon or metal spatula to press the edges of the cookies into the center. And after that, press a few chocolate chips into the tops of the cookies -- this not only ensures chocolate in every bite, it also makes them look pretty!
ENJOY: Allow cookies to stand on sheet pan for 5 minutes before removing them to a wire cooling rack. Repeat this process to bake all of the cookies. These cookies taste best at room temperature and even better the next day -- the pumpkin flavor has intensified by then!