Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.

In a medium bowl, whisk together the flour, cornstarch, baking soda, cream of tartar, baking powder, salt, and cinnamon (or pumpkin pie spice).

In a large mixing bowl, combine the butter, brown sugar, granulated sugar, pumpkin purée, and vanilla, mix until smooth.

Add the dry ingredients to the wet mixture and stir just until combined. Fold in oats and any add-ins.

Cover and refrigerate for 30–45 minutes. This step is key for chewy cookies!

Scoop tablespoons of dough (or use a medium cookie scoop) onto the prepared sheets, spacing about 1 ½ inches apart, and press down slightly to flatten. Bake 11–13 minutes, until edges are set but centers still look soft.

Let cookies cool on the pan for 5 minutes before transferring to a wire rack.