Preheat oven to 350°F (180°C). Line a 13x9-inch baking pan with parchment paper, letting excess extend over sides of pan.
In a large bowl, whisk together brown sugar, melted butter, and granulated sugar until well combined. Add pumpkin, eggs, and Pumpkin Spice Vanilla Extract, stirring until combined.
In a medium bowl, sift together flour, baking powder, baking soda, salt, cinnamon, nutmeg, ginger, cloves, and allspice. Gradually add flour mixture to sugar mixture, whisking just until combined. (Batter will be smooth.) Pour batter into prepared pan. Using a mini offset spatula, smooth batter into an even layer. Run the tip of a knife through batter to help release any large air bubbles. Tap pan on counter 2 to 3 times to level batter.
Bake until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Let cool completely in pan. Spread Pumpkin Spice Cream Cheese Frosting in an even layer on top of cooled bars. Cover and refrigerate until frosting is set, at least 30 minutes. Using excess parchment as handles, remove from pan, and cut into bars. Garnish with cinnamon and pecans, if desired.
Recipe by Bake from Scratch at https://bakefromscratch.com/vanilla-pumpkin-spice-bars/