Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.

Make peanut butter batter: Beat cream cheese, peanut butter, sugar, egg, and milk in a medium bowl until smooth. Set aside.

Make brownie batter: Mix together sugar, melted butter, and vanilla in a large bowl until combined. Mix in eggs one at a time, beating well after each addition. Combine flour, cocoa, baking powder, and salt in a separate bowl; mix into batter until well combined. Stir in chocolate chips until incorporated.

Set aside 1 cup brownie batter. Spread remaining brownie batter into the prepared dish. Spread peanut butter batter on top. Drop reserved chocolate batter by teaspoonfuls over the top. Using a knife, gently swirl through the top layers for a marbled effect.

Bake in the preheated oven until a wooden toothpick inserted near the center comes out almost clean, 35 to 40 minutes. Cool completely, then cut into bars.