1. Cut orange in half crosswise. Run a sharp knife or grapefruit knife around each section to loosen from membrane. (Do not remove sections.)
2. Stir together melted butter, chopped mint, and, if desired, orange liqueur. Spoon evenly over cut sides of orange halves. Place orange halves, cut side up, on an aluminum foil-lined baking sheet.
3. Broil 5 inches from heat 5 to 8 minutes or until thoroughly heated.