Preheat oven to 350°. Grease one 9x13-inch baking pan. In a medium bowl, beat cream cheese, peanut butter, 1/4 C. white sugar, 1 egg, and milk until smooth. Set aside. In a large bowl, mix together melted butter, 2 C. white sugar, and vanilla.
Mix 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 C. 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 affect. Bake in preheated oven for 35-40 minutes, or until a wooden toothpick inserted near the center comes out almost clean. Cool completely, then cut into bars.
Yields: 36 servings.