Heat oven to 350°F. Grease 9-inch square baking pan. Stir together butter, sugar and vanilla in bowl. Add eggs; beat well with spoon. Stir together flour, cocoa, baking powder and salt; gradually add to egg mixture, beating until well blended. Stir in nuts, if desired. Spread batter evenly in prepared pan. Bake 20 to 25 minutes or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack.