Preheat oven to 350°F. Grease a 13x9€? pan and set aside. In a large, microwavable bowl, combine baking chocolate and butter, then microwave on high for about 2 minutes. Remove from microwave and stir until chocolate is completely melted and mixture is smooth. Mix sugar into chocolate mixture and then slowly add mayonnaise, eggs and vanilla extract. Stir in flour and baking powder. Add chocolate chips and nuts, if desired. Pour batter into pan and smooth top. Bake for about 30-35 minutes. Remove from oven and cool completely before cutting. Dust brownies with powdered sugar if desired.
