Preheat oven to 350°F. Grease an 8x8x2-inch or 9x9x2-inch baking pan; set aside. In a medium saucepan, combine butter and bittersweet chocolate. Cook and stir over medium heat until butter and chocolate are melted. Remove from heat. Using a wooden spoon, stir granulated sugar and brown sugar into chocolate mixture until smooth. Add eggs, beating with spoon until well combined. In a small bowl, combine flour and baking powder. Stir flour mixture into the chocolate mixture just until combined. Stir in nuts and milk chocolate pieces (batter will be thick). Spread in prepared pan. Bake for 25 minutes or until top is set but still soft. (A toothpick inserted in center will come out slightly gooey.) Cool on a wire rack. Cut into 16 brownies.