1. Heat oven to 325 degrees F. Coat 13 x 9 x 2-inch baking pan with nonstick cooking spray.Brownies:
2. Beat together the butter, cream cheese and sugar on medium-high speed in medium-size bowl for about 2 minutes or until mixture is smooth. Scrape down sides of bowl; beat for another 1 minute. Beat in the eggs and vanilla; beat 2 minutes.
3. Stir together the flour, cocoa powder, salt and baking powder in another bowl.
4. On low speed, gradually beat the flour mixture into the butter mixture; beat on medium-high speed for 2 minutes. Stir in peanut butter chips. Spoon batter into prepared pan, spreading evenly.
5. Bake in 325 degrees F oven for 35 to 40 minutes or until brownies begin to pull away from sides of pan. Let cool in pan on wire rack.Frosting:
6. Beat together the butter and cocoa powder in a small bowl until blended. Gradually beat in the confectioners sugar alternately with the milk and vanilla until well blended.
7. Spread frosting over cooled brownies. Refrigerate for at least 1 hour. Cut into 36 brownies.