Preheat oven to 325 degrees.
In a medium saucepan melt butter or margarine and unsweetened chocolate over low heat.
Transfer chocolate to a large mixer bowl.
Add sugar, mix well. Add eggs one at a time, beating just until blended.
In a mixing bowl stir together flour and baking powder.
Add to chocolate mixture along with vanilla, mix well.
Pour batter into a prepared 13x9 inch baking pan.
Sprinkle with chopped walnuts and chocolate pieces.
Bake about 45 minutes or until toothpick inserted in center comes out clean.
Cool completely on wire rack. Cut into bars.