Preheat your oven to 350°F (175°C). Grease an 8×8-inch square baking pan or line it with parchment paper, leaving an overhang on the sides for easy removal of the blondies later.

In a large mixing bowl, whisk together the melted butter and light brown sugar until smooth. Add the pumpkin puree, egg, and vanilla extract. Whisk vigorously until the mixture is well combined and slightly lightened in color.

In a separate medium bowl, sift together the all-purpose flour, baking powder, cream of tartar, pumpkin pie spice, and salt. Stir gently to ensure the leavening agents are evenly distributed.

Gradually add the dry ingredients to the wet ingredients. Use a spatula to fold the mixture together until just combined. Be careful not to overmix, as this can make the blondies tough instead of chewy.

Transfer the batter into the prepared baking pan and spread it into an even layer with a spatula. In a small bowl, mix the granulated sugar and cinnamon for the topping. Sprinkle this mixture evenly over the top of the batter.

Place the pan in the center rack of the oven. Bake for 25 to 30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with just a few moist crumbs attached.

Remove the pan from the oven and set it on a wire rack to cool completely. Allowing them to cool in the pan is essential for the texture to set properly. Once cool, lift them out using the parchment paper and cut into 16 squares.