Heat oven to 325°F. Coat 13 x 9 x 2-inch baking pan with nonstick cooking spray.
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.
Stir together the flour, cocoa powder, salt and baking powder in another bowl.
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.
Bake in 325°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.
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.
Spread frosting over cooled brownies. Refrigerate for at least 1 hour. Cut into 36 brownies.