In a mixer bowl on low speed, combine cream cheese, peanut butter, and sugar until smooth. Add egg, milk and vanilla. Continue beating until well mixed. Use a spatula to fold in peanuts. Set aside.
Heat oven to 350°F. Butter a 9-inch square pan. In a heavy saucepan melted chocolate chips and butter over very low heat. Stir until melted. Remove from heat. Set aside.
In a bowl sift together flour, cocoa, baking powder and salt. Combine sugar, eggs and vanilla in a mixer bowl and beat until smooth and lemon-colored, about 2 minutes. Beat in melted chocolate mixture until blended. Add flour mixture and continue beating until mixture is smooth. Spread half the brownie batter in prepared pan. Carefully spread filling over chocolate, then gently spoon remaining batter over filling. Spread batter lightly to cover filling. Bake 30-40 minutes or until center is just set. Cool on a wire rack before cutting into 2-inch squares.