Preheat oven to 180ºC . Butter a 9 inchx13 inch baking dish. Melt together the chocolate squares and chips, and butter. Cool Beat together eggs, sugar and vanilla. Beat in chocolate mixture. Mix flour, salt and baking powder. Stir into batter. Add marshmallows and walnuts. Pour into pan. Bake 40-45 minutes. Do not overcook. Brownies will become firm when cool. Makes 30-36 squares.