Preheat oven to 350°F. Line a 9×9-inch baking dish with parchment paper. Set aside.
Whisk melted butter and brown sugar in a large bowl until fully incorporated.
Add the pumpkin and vanilla. Whisk to combine.
In a separate, medium-sized bowl, whisk together the flour, cornstarch, pumpkin spice, baking powder, salt, and nutmeg.
Gradually stir the flour mixture into the pumpkin mixture until completely combined.
Spread the batter into the bottom of the lined baking dish. (The batter is thick.)
In a small bowl, whisk together the granulated sugar and cinnamon.
Evenly sprinkle the mixture over the batter.
Bake for 35-38 minutes, or until the edges just begin to turn golden brown and the center is no longer jiggly. (Don't overbake! Check at 30 minutes - if an inserted toothpick is removed with crumbs but no wet batter it is done. The goal is "fudgy" pumpkin brownies)
Allow the bars to cool slightly before cutting and serving.