Preheat oven to 350°F (175°C). Foil-line 8x8-inch pan; spray.

Prepare brownie batter per box; reserve about 1/2 cup for topping.

Beat cream cheese, sugar, egg, and vanilla smooth.

Spread most brownie batter in pan; dollop cheesecake mixture over and spread gently.

Spoon reserved brownie batter over cheesecake; swirl with knife.

Sprinkle optional chocolate chips or nuts. Bake 30–35 minutes until set but center jiggles slightly.

Cool completely; refrigerate 1–2 hours, lift out, and cut bars.