Heat oven to 350F. Grease 13x9x2inch pan. In a large microwave safe bowl, place butter. Microwave at High 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 until well blended. Add flour, baking powder and salt; beat well. Stir in nuts if desired. Pour batter into prepared pan. Bake 30 to 35 or until brownies pull away from the sides of the pan. Cool. Cut into bars. About 35 brownies