Heat oven to 350°F. Grease 9" 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 into prepared pan.
Bake 15 to 20 minutes or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack.
Meanwhile, combine butter, cocoa, corn syrup, milk and vanilla extract in a medium bowl. Beat with electric mixer on slow speed. Slowly add icing sugar until well combined. Spread on cooled brownies.
Makes 10-16 brownies