Preheat oven to 350°F (177°C)
In a nonstick pan add pumpkin puree. Cook the puree on low heat until almost all the moisture has evaporated( around 8-10 mins). At the end, the total weight of the puree should be 115 gms-120 gms. It should be really thick. (ref note 1).
Cool down the pumpkin puree completely.
In a bowl mix flour, oats, baking soda, pumpkin spice, and cinnamon.
In another bowl add melted butter, brown sugar, and white sugar. Whisk it well.
Add vanilla extract, egg yolk, and pumpkin puree. Whisk it well.
Fold in the dry mixture and chocolate chip. Use a spatula to mix everything.
Using a medium cookie scoop, scoop cookie dough into balls (about 2 heaping Tbsp of dough each) and place 3 inches apart on the cookie sheet. Slightly flatten the balls with the back of the spatula.
Bake it for 14-15 mins. Check once at 12 minutes, if the cookies have browned and the edges look firm and settled remove the tray.
Let the cookies rest on the tray for 10 minutes, for the cookies to settle. Then place it on a wired rack to cool completely.
