Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on the sides, and lightly grease the paper.
In a large bowl, whisk melted butter, granulated sugar, and brown sugar until smooth. Add eggs one at a time, whisking well after each, then stir in vanilla.
In a separate bowl, whisk flour, cocoa powder, and salt. Gradually add dry ingredients to wet mixture, stirring just until combined. Fold in chocolate chips if using.
In another bowl, beat softened cream cheese with a hand mixer until smooth. Add sugar and beat until combined. Mix in pumpkin puree, egg, vanilla, pumpkin pie spice, and salt until smooth.
Pour about two-thirds of brownie batter into the pan and spread evenly. Set aside remaining one-third. Pour pumpkin cheesecake batter over and gently spread.
Drop spoonfuls of reserved brownie batter over the cheesecake layer. Use a butter knife to swirl in a figure-eight or zigzag pattern for a marbled effect.
Bake for 35 to 45 minutes until the cheesecake layer is set and a toothpick inserted in the brownie portion comes out with moist crumbs. Center may be slightly jiggly.
Cool completely in the pan at room temperature for 1 hour, then refrigerate for at least 2 hours until fully set before slicing.
Use parchment overhang to lift the slab out of the pan. Slice into 24 squares with a sharp knife, wiping clean between cuts.