Preheat oven to 350 degrees F (175 degrees C). Grease one 9x13 inch baking pan.
In a medium bowl, beat cream cheese, peanut butter, 1/4 cup white sugar, 1 egg, and milk until smooth. Set aside.
In a large bowl, mix together melted butter, 2 cups white sugar, and vanilla. Mix in the remaining 3 eggs one at a time, beating well after each addition. Combine flour, cocoa, baking powder, and salt; mix into the batter. Stir in chocolate chips.
Remove 1 cup of the chocolate batter. Spread the remaining batter into the prepared pan. Spread the peanut butter filling over the top. Drop the reserved chocolate batter by teaspoonful over the filling. Using a knife, gently swirl through the top layers for a marbled effect.
Bake in preheated oven for 35 to 40 minutes, or until a wooden toothpick inserted near the center comes out almost clean. Cool completely, then cut into bars.