Heat oven to 350 degrees Fahrenheit. Grease 13x9x2-inch baking pan. In large microwave-safe bowl, place chocolate and butter. Microwave at HIGH (100%) 1-1/2 to 2 minutes or until chocolate is melted and mixture is smooth when stirred. Add sugar; stir with spoon until well blended. Add eggs and vanilla; mix well. Add flour and nuts, if desired; stir until well blended. Spread into prepared pan. Bake 30 to 35 minutes or until wooden pick inserted in center comes out almost clean. Cool completely in pan on wire rack. Frost with QUICK & EASY CHOCOLATE FROSTING, if desired. Cut into squares. About 24 brownies. Quick & Easy Chocolate Frosting In medium saucepan over low heat, melt chocolate, stirring constantly. Add marshmallows; stir frequently until melted. (Mixture will be very thick and will pull away from sides of pan). Spoon mixture into small mixer bowl; beat in butter. Gradually add milk, beating until smooth. Add powdered sugar and vanilla; beat to desired consistency. About 2-1/4 cups frosting. Hershey's is a registered trademark of Hershey Foods Corporation. Recipe may be reprinted courtesy of the Hershey Kitchens.