1. Heat oven to 350°F. Grease 13x9x2-inch baking pan.
2. Place butter in large microwave-safe bowl. 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 until well blended. Add flour, baking powder and salt; beat well. Stir in nuts, if desired. Pour batter into prepared pan.
3. Bake 30 to 35 minutes or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack. Frost if desired. Cut into bars.
