Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper (or lightly grease it with cooking spray) for easy removal.
In a large mixing bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth and glossy.
Add in the pumpkin puree, egg yolk, and vanilla extract. Whisk until fully combined.
In a separate bowl, mix the flour, baking soda, cream of tartar, cinnamon, nutmeg, and salt.
Combine the wet and dry ingredients until everything is just combined—don’t overmix! The batter will be thick, almost like cookie dough.
Transfer the dough into your prepared pan and use a rubber spatula to spread it evenly into the corners.
In a small bowl, mix together the 2 tablespoons of sugar and 1 teaspoon of cinnamon (or more! I didn’t really measure mine), then sprinkle the mixture evenly over the top.
Bake for 25–30 minutes, or until the edges are lightly golden and the center is set. A toothpick inserted near the center should come out with just a few moist crumbs.
Let the bars cool in the pan for at least 20–30 minutes before slicing.