Heat oven to 325°. Line a 9 x 9-inch baking pan with foil. Coat foil with nonstick cooking spray.
In a small heavy saucepan, heat butter and chocolate over low heat, stirring constantly, until chocolate is melted and smooth, about 5 minutes. Set aside to cool. In a small bowl, stir flour, baking powder and salt. Set aside.
In a large mixing bowl, beat eggs and sugar with an electric mixer on high speed 5 minutes or until lemoncolored and fluffy, occasionally scraping sides of bowl. Add cooled chocolate mixture and vanilla. Beat on low speed until combined. Add flour mixture. Beat on low speed until combined, scraping sides of bowl. Stir in 1/2 cup of the Oreo pieces. Spread batter into prepared pan and scatter remaining cookie pieces on top.
Bake at 325° for 30 minutes or until brownie appears set. Cool in pan on a wire rack. Once cool, use foil to lift brownie from pan. Cut into 9 squares.