Heat oven to 325°F. Line bottom and sides of 9-inch square pan with foil, extending foil 2 inches on 2 opposite sides of pan; spray foil with cooking spray.
In medium bowl, beat melted 3/4 cup butter, the granulated sugar and orange peel with wooden spoon until blended. Beat in eggs, one at a time. Stir in flour and baking powder just until mixed. Stir in yellow food color until well mixed. Spread 1 1/2 cups yellow batter in pan. Place pan in freezer about 15 minutes or until batter is slightly firm to the touch.
Meanwhile, add red food color to remaining cake batter in bowl until well mixed. Spread over chilled yellow batter.
Bake 47 to 53 minutes or until toothpick inserted in center comes out clean. Cool completely, about 45 minutes.
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.