Preheat the oven to 350℉. Spray a 9X9-inch square pan with baking spray. Line the bottom and all 4 sides with parchment paper. Spray again.
In a medium bowl, sift the flour, Add the cornstarch, baking powder, baking soda, and salt. Set aside.
1 1/4 cups All-purpose flour, 1 tsp Cornstarch, 1/4 tsp Baking powder, 1/4 tsp Baking soda, 1/2 tsp Salt
In a separate bowl using a mixer, beat the melted butter and sugar on high speed for 2 minutes.
1/2 cup Unsalted butter, 3/4 cup White granulated sugar
Add in the vanilla and egg and mix on medium until combined. Add in the dry ingredients and mix on low speed until just combined.
1 Large egg, 1 tsp Pure vanilla extract
Take the cinnamon bowl out of the fridge. Use a fork or your hands to break up the clumps. Add the cinnamon filling to the cookie dough. Mix on low speed until mixed together.
Pour the cookie dough into the pan. Use your hands to press the dough evenly on to the bottom of the pan.
Bake for 19-22 minutes. Bake until the cookie is lightly golden brown. Cool completely before adding the pumpkin batter.