Preheat the oven to 350°F. Line a 9 x 13-inch baking pan with parchment paper, leaving an overhang on two sides. Lightly coat the paper with nonstick cooking spray.

Mix the wet ingredients

In a large bowl, whisk the melted butter and sugar until combined. Add the eggs one at a time, whisking after each addition. Mix in the vanilla extract.

Finish the brownie batter

Add the flour, cocoa powder, salt, and baking powder. Fold gently with a rubber spatula until only a few dry streaks remain. Fold in the chocolate chips without overmixing.

Fill the pan

Spread the brownie batter evenly in the prepared baking pan. Reserve approximately 2 tablespoons of batter for small decorative swirls if desired.

Prepare the cheesecake mixture

In a medium bowl, beat the softened cream cheese and sugar until smooth. Add the egg yolk, vanilla, and flour. Beat only until evenly combined.

Create the ghosts

Drop 12 to 16 rounded spoonfuls of cheesecake mixture over the brownie batter. Use the back of a small spoon to gently pull the bottom of each dollop into a short ghost tail. Keep space between the ghosts so they remain visible after cutting.

Bake the brownies

Bake for 28 to 33 minutes. The edges should look set, while the center should remain slightly soft. A toothpick inserted into a brownie area should come out with moist crumbs, not raw batter.

Cool completely

Place the pan on a wire rack and allow the brownies to cool completely for approximately 90 minutes. Use the parchment-paper overhang to lift them from the pan.

Add the ghost faces

Melt the chocolate chips in the microwave in 15-second intervals, stirring after each interval. Add the optional oil if the chocolate is too thick. Transfer the chocolate to a small piping bag or zip-top bag and cut a tiny opening. Pipe two eyes and a small mouth onto each cheesecake ghost.

Cut and serve

Allow the chocolate faces to set. Cut the brownies into 16 squares with a clean sharp knife, wiping the blade between cuts. Serve at room temperature or slightly chilled.