Line a 15x10x1-inch baking pan with foil, extending the foil over edges of pan. Grease foil; set aside.
In a medium saucepan, combine chocolate and butter. Heat and stir constantly over low heat until melted and smooth. Remove from heat. Let cool for 10 minutes.
Meanwhile, in a small bowl, stir together flour and baking powder; set aside.
In a large mixing bowl, combine sugar, eggs and vanilla. Beat with an electric mixer on medium to high speed for 2 to 3 minutes or until color lightens slightly. Beat or fold in melted chocolate mixture. Add flour mixture to chocolate mixture; stir just until combined. Spread the batter evenly into prepared pan.
Bake in a 350° oven for 25 minutes. Cool in pan on a wire rack. Use foil to lift uncut brownies out of pan. Place on cutting board; cut into bars. If you like, top brownies with raspberries or sliced strawberries and whipped cream.