Preheat oven to 350°F. Line a 9x13” pan with foil and spray with nonstick cooking spray.
Place melted butter, pumpkin puree, brown sugar, and granulated sugar in the bowl of a strand mixer fitted with the paddle attachment. (You can also use a hand mixer or a wooden spoon.) Mix until combined. Continue mixing and add the egg yolk and vanilla extract. Add the baking soda, salt, nutmeg, cinnamon, ginger, and allspice and mix until combined, scraping the sides of the bowl as needed. Slowly mix in flour until a smooth dough forms.
Press dough into prepared pan. Bake for about 15-20 minutes or until just browned around the edges and the teeniest bit jiggly in the center (these are better slightly under baked as they finish baking while they cool).
Make the frosting: beat cream cheese and butter until smooth. Add vanilla and mix in powdered sugar, 1 cup at a time, until smooth. Slowly mix in cream until you get a spreadable consistency.
Frost the bars as desired. Store in an airtight container in the refrigerator for up to 3 days or freeze in a single layer for up to one month.