Heat oven to 350'F. Grease 13x9x2" baking pan. In large microwave-safe bowl, place butter. Microwave at High (100%) 2 to 2 1/2 minutes or until melted. Stir in sugar and vanilla. Add eggs, one at a time, beating well with spoon after each addition. Add cocoa; beat well until blended. Add flour, baking powder and salt; beat well. Stir in nuts, if desired. Pour batter into prepared pan. Bake 30-35 minutes or until brownies begin to pull away from sides of pan. Cut into bars. Makes about 36 brownies.