Line an 8x8" pan with parchment paper and preheat the oven to 350℉/180℃.
In a bowl, whisk together the flour, baking powder, cinnamon and salt and set aside.
1⅓ cups all-purpose flour, 1¼ teaspoons baking powder, 1 teaspoon ground cinnamon, ½ teaspoon kosher salt
In a large mixing bowl, whisk together the sugar, egg yolk and melted butter.
¾ cup granulated sugar, 1 large egg yolk, ½ cup unsalted butter
Add the flour mixture to the wet ingredients and use a rubber spatula to fully combine the batter until no streaks of flour remain and it becomes thick.
Transfer the cake batter into the pan and press it evenly onto the bottom, getting it the whole way to the corners.
In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), beat together the cream cheese and powdered sugar on low until combined, then increase the speed to medium for about a minute.
6 ounces full-fat cream cheese, 1½ cups powdered sugar
Scrape down the sides, then add the egg, pumpkin, vanilla, and pumpkin pie spice and beat until combined and smooth.
1 large egg, ½ cup pure pumpkin puree, 1 teaspoon vanilla paste or extract, 1 teaspoon pumpkin pie spice
Pour the pumpkin batter on top of the spice cake and spread evenly.
Bake the bars in the center rack of the oven for 25-30 minutes until the edges are set, but the center is still slightly jiggly (but not sloshy).
Remove the pan and cool on a wire rack for one hour, then transfer the pan to the fridge (uncovered) for 3-4 hours.
Cut the bars with a large sharp knife, wiping off the blade in between each cut to get very clean slices.
Use a fine mesh strainer to dust the tops of the bars with powdered sugar (optional).