01 - Preheat oven to 350°F. Line a 9x9-inch baking pan with parchment paper, leaving an overhang to assist with removal.
02 - In a large bowl, whisk melted butter and granulated sugar until well combined. Add eggs and vanilla extract, whisking until completely smooth.
03 - Sift flour, unsweetened cocoa powder, salt, and baking powder directly into the wet mixture. Fold together gently, just until integrated to avoid overmixing.
04 - Spread approximately three-quarters of the brownie batter evenly into the prepared pan.
05 - In a separate bowl, beat cream cheese until smooth using an electric mixer or whisk. Add granulated sugar, egg, and vanilla extract, mixing until creamy. Tint the mixture with green gel food coloring to reach a vibrant hue.
06 - Dollop the green cheesecake mixture onto the layered brownie batter. Place the remaining brownie batter in small spoonfuls over the surface.
07 - Use a knife or skewer to gently swirl the cheesecake and brownie batters together, forming a marbled pattern.
08 - Top with candy eyeballs and Halloween sprinkles if desired.
09 - Transfer to oven and bake for 33 to 36 minutes, or until the center is just set and a toothpick inserted shows a few moist crumbs.
10 - Allow brownies to cool completely in the pan. Lift out using the parchment overhang, then slice into squares.