I got these from the kitchn and they are addictive! The recipe is easily doubled (just use a 10"x13" pan), and they will go fast!
½ C butter
½ C plus 3 T sugar, divided
2 ½ tsp pumpkin pie spice, divided
½ C packed brown sugar
½ C pumpkin puree
1 T vanilla extract
¼ tsp kosher salt
1 tsp cream of tartar
¼ tsp baking powder
¼ tsp baking soda
1 1/3 C flour
Preheat oven to 350°F.
Put butter in a large microwave-safe bowl. Microwave until mostly melted, 50-60 seconds (it will melt completely as it sits). (Alternatively, melt the butter in a small saucepan over medium heat.) Set aside for 5 minutes to cool slightly.
Meanwhile, line an 8"x8" metal pan with a sheet of parchment paper, leaving an overhang on two opposite sides to form a sling. Lightly coat the pan and parchment with cooking spray or butter.
Place 3 T of the sugar and 2 tsp of the pumpkin pie spice in a small bowl and whisk to combine.
Add the remaining ½ C sugar, brown sugar, pumpkin, vanilla extract, and salt to the butter. Stir with a flexible spatula until smooth and glossy.
Evenly sprinkle the remaining ½ tsp pumpkin pie spice, cream of tartar, baking powder, and baking soda over the mixture. Add the flour and stir with the spatula until a thick batter forms.
Transfer the batter to the pan and spread into an even layer. Sprinkle the cinnamon-sugar mixture evenly over the top. Lightly press the sugar into the batter with your fingers or the spatula.
Bake for 15 minutes, rotate the pan, and bake another 13-15 minutes more, until the edges pull away slightly from the pan and are firm, and a tester inserted into the center comes out with just a few crumbs.
Cool completely in pan on a wire rack, about 2 hours. Use the overhanging parchment to lift the slab out of the pan and onto a cutting board, and cut into pieces as desired.