Prep the pan and oven: Heat the oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter: In a medium saucepan over low heat, melt the butter. Remove from heat and whisk in granulated and brown sugars until glossy and combined.

Whisk in eggs and vanilla: Add the eggs one at a time, whisking well after each. Stir in the vanilla. The mixture should look thick and shiny.

Fold in dry ingredients: Sift in cocoa, flour, and salt. Switch to a spatula and fold just until no dry streaks remain. Fold in mini chocolate chips, if using. Do not overmix.

Spread batter: Pour the brownie batter into the prepared pan and smooth the top. Tap the pan once on the counter to level.

Make the cheesecake mixture: In a bowl, beat the softened cream cheese with sugar until smooth and creamy. Add the egg yolk, vanilla, salt, and cornstarch. Beat just until silky and thick.

Bag it up: Transfer the cheesecake mixture to a piping bag fitted with a round tip (or use a zip-top bag with a 1/2-inch corner snipped off).

Pipe the ghosts: Pipe small mounds of cheesecake on top of the brownie batter, using a steady squeeze and small upward pull to create a ghostly peak. Leave a little space between ghosts to help them hold shape.

Add faces: Gently press mini chocolate chips point-side in for eyes. For a mouth, add another chip or a tiny sprinkle of cocoa.

Bake: Bake for 28–34 minutes, until the brownie edges are set and a toothpick inserted 2 inches from the edge comes out with a few moist crumbs. The cheesecake ghosts should be just set with a slight wobble.

Cool completely: Let the pan cool on a rack for 1–2 hours. For clean cuts and best flavor, chill at least 1 hour after cooling.

Slice and serve: Lift out using the parchment. Warm a sharp knife under hot water, wipe dry, and cut into squares. Serve slightly chilled or at room temperature.