In medium bowl, beat 1/2 cup softened butter with electric mixer on medium speed until light and fluffy. Beat in powdered sugar until well blended. Beat in milk, vanilla and salt until smooth and spreadable. Spread frosting over bars. To serve, remove bars from pan, using foil to lift. Remove foil. Cut into 4 rows by 4 rows; cut each square diagonally in half. Top each triangle with 1 candy corn.