Preheat the oven to 350°F. Line 2 baking sheets with parchment paper.

In a medium bowl, whisk together the flour, baking soda, pumpkin pie spice, cinnamon, salt, and cornstarch.

In a large bowl, whisk the melted butter, brown sugar, and granulated sugar until smooth and glossy.

Whisk in the pumpkin puree, egg yolk, and vanilla extract until fully combined.

Add the dry ingredients to the wet ingredients and stir with a spatula just until no dry streaks remain.

Fold in the chocolate chips or chunks. The dough will be soft but should hold its shape.

Chill the dough for 30 minutes to help the cookies bake up thick.

Scoop the dough into 2-tablespoon portions and place them a few inches apart on the prepared baking sheets.

If desired, press a few extra chocolate chips onto the tops of the dough balls.

Bake for 11 to 13 minutes, until the edges are set and lightly golden and the centers still look slightly soft.

Sprinkle lightly with flaky sea salt, if using. Let the cookies cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.