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.
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 (55 grams) each. (See Note 4; batter should make 17-18 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. (If cookies are poofy, 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.
ENJOY: Allow cookies to stand on the 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!
OPTIONAL GLAZE: Add milk to a medium pot, heat over low heat until warmed. Add powdered sugar and whisk in slowly until well combined. Remove glaze from heat and stir in pumpkin pie spice and vanilla. Dip completely cooled cookies into the glaze, one at a time, and set on a cooling rack atop a sheet pan. Let stand 5 minutes until glaze is hardened. (If glaze starts to harden while dunking cookies, just whisk it up again!) Heads-up: You'll likely have a bit extra glaze, but dipping works best with this quantity.