Add room-temperature butter, brown sugar, and granulated sugar to a stand mixer with a whisk attachment. Beat until creamy, scraping edges as needed. Add pumpkin, egg yolk, and vanilla. Mix to incorporate.
Add cinnamon, pumpkin pie spice, baking soda, baking powder, salt, and quick oats to the mixture. Beat until combined. Add flour and mix until no streaks remain.
Cover and place the bowl of dough in the fridge for 1 hour.
Roll chilled dough into large, 3-tablespoon balls or 55 grams (see note 4). Roll to be taller instead of wider and place on a parchment-lined tray or plate. Batter should make 17–18 cookies. Chill dough balls in the fridge for 15–20 minutes. While dough balls are chilling, preheat oven to350℉. Line a large sheet pan with parchment paper or a silicon baking mat.
Add only 6 dough balls to a sheet pan at a time, spacing them out well. Bake 12–15 minutes or until very lightly browned at the edges and slightly gooey in the center. These cookies are best slightly underbaked! If cookies are poofy, right out of the oven, bang the sheet pan on the counter a few times to flatten cookies slightly.
Optionally, right after banging the pan, quickly press cookie edges inward with the back of a large spoon.
Let cookies rest on the pan 5 minutes before transferring to a cooling rack. Repeat this process to bake all cookies. These cookies taste best at room temperature and even better the next day—the pumpkin flavor has intensified by then!
Add milk to a medium pot and 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!) You'll likely have a bit extra glaze, but dipping works best with this quantity.
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Notes
Note 1: It’s really important to use butter at room temperature so it will cream with the sugars properly. Don’t melt or soften; set it out at room temperature for 45 minutes to an hour before starting.Note 2: Although it would make sense that any oats would work, they aren’t interchangeable here. Quick oats act more like flour since they are smaller, denser, and more compact.Note 3: Pumpkin pie spice can be found among other spices in the grocery store, or you can make your own pumpkin pie spice!Note 4: Cookies work best at this size. While you may want smaller cookies, know that the texture is not the same. The sweet spot for this recipe is exactly 55 grams (3 packed tablespoons) of dough. Big, I know, but totally perfect for ensuring crisp edges and soft, chewy center!Storage: Most cookies are the best right out of the oven, but these are best at room temperature—and even better (with a much more pronounced pumpkin flavor) the next day! I actually don’t cover these cookies—just leave them on a plate at room temperature—this keeps them from becoming too moist and losing texture. After baking the cookies, let them cool completely on a wire rack (this will keep the bottoms from getting soggy from the steam).