Preheat oven to 350°F (175°C). Line 8x8-inch pan with foil and spray.
Prepare brownie batter per box directions. Reserve about 1/2 cup batter for topping.
Beat cream cheese, sugar, egg, and vanilla until smooth.
Spread most of the brownie batter into pan. Dollop cheesecake mixture over and spread gently.
Spoon reserved brownie batter over cheesecake and use a knife to create swirls.
Sprinkle chocolate chips or nuts if using. Bake 30–35 minutes until set but slightly jiggly in center.
Cool completely, refrigerate 1–2 hours, then lift out and cut into bars.