Preheat oven to 350°F (175°C). Line an 8x8-inch square metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting. Lightly grease parchment with butter.

In a heatproof bowl set over a pot of barely simmering water (or in microwave in 30-second bursts), melt the chopped dark chocolate and butter together until smooth and glossy. Remove from heat and let cool for 5 minutes.

In a large mixing bowl, beat the 3 eggs, granulated sugar, and brown sugar vigorously with a hand mixer or whisk for 2 to 3 minutes until pale, thickened, and foamy (this dissolved sugar-egg emulsion is the key to that signature shiny crinkly top crust!). Whisk in vanilla and the melted chocolate mixture.

Gently fold in the flour, cocoa powder, and fine sea salt with a rubber spatula just until combined (do not overmix). Fold in chocolate chips. Pour about 3/4 of the brownie batter into the prepared baking pan and smooth the surface.

Prepare the pumpkin cheesecake swirl: In a medium bowl, beat the softened cream cheese, blotted pumpkin puree, sugar, egg yolk, cinnamon, pumpkin pie spice, and vanilla until smooth and creamy.

Dollop spoonfuls of the spiced pumpkin cheesecake mixture over the brownie batter. Drop the remaining 1/4 of brownie batter in small dollops between the pumpkin. Take a wooden skewer or butter knife and swirl the batters together in gentle figure-eight motions to create an intricate marbled pattern.

Bake for 32 to 36 minutes at 350°F until the edges are set and a toothpick inserted in the center comes out with a few moist fudgy crumbs (not wet batter). Transfer to a wire rack and cool completely to room temperature, then chill in the refrigerator for at least 2 hours before slicing into clean, sharp squares.