Prep the pan: Heat oven to 350°F (175°C). Line an 8x8-inch pan with parchment, leaving overhang for easy lifting. Lightly grease.

Make the brownie base: In a microwave-safe bowl, melt the butter. Whisk in sugar until glossy. Add eggs and vanilla; whisk 30–45 seconds until thickened slightly.

Whisk in dry ingredients: Sift in cocoa, flour, salt, and baking powder. Stir just until no dry spots remain. Fold in chocolate chips if using. Reserve about 1/3 cup of batter for swirling and spread the rest in the pan.

Make the cheesecake layer: In a medium bowl, beat softened cream cheese until smooth. Mix in pumpkin purée, sugar, egg yolk, vanilla, pumpkin spice, and a pinch of salt until creamy with no lumps.

Layer and swirl: Spoon the pumpkin cheesecake mixture evenly over the brownie batter. Dollop the reserved brownie batter in 6–8 spots on top. Use a knife or skewer to gently swirl—aim for figure-eights without overmixing.

Bake: Bake 30–36 minutes, until the edges are set and the center has a slight jiggle but doesn’t look wet. A toothpick should come out with a few moist crumbs from the brownie portion.

Cool completely: Set the pan on a rack. For neat slices, chill at least 1 hour after cooling to room temp. Lift out with the parchment and cut into 9 or 16 squares.

Finish and serve: Sprinkle a little flaky salt if you like. Serve at room temperature for the best texture and flavor.