Preheat the oven to 350°F. Line a 9×9-inch baking dish with parchment paper and set aside.

In a large bowl, whisk the melted butter and brown sugar until fully combined.

Add the pure pumpkin and vanilla extract. Whisk until smooth.

In a separate medium bowl, whisk together the flour, cornstarch, pumpkin pie spice, baking powder, kosher salt, and nutmeg.

Gradually stir the dry ingredients into the wet mixture until no dry streaks remain. The batter will be thick.

Spread the batter evenly into the prepared pan.

In a small bowl, stir together the granulated sugar and cinnamon.

Sprinkle the mixture evenly over the batter.

Bake for 35 to 38 minutes, until the edges are lightly golden and the center is set but still soft. A toothpick inserted in the center should come out with a few moist crumbs. Do not overbake.

Cool in the pan for 20 to 30 minutes before lifting out and cutting into bars.