Add 2 ounces of Limoncello, 2 ounces of Domaine de Canton, 1 tablespoon lemon juice, and ½ cup ice to the cocktail shaker. Shake for 10 seconds, then strain mixture and divide evenly among 4 regular or sugar rimmed glasses. Top off each glass with Champagne and add a lemon slice. Serve immediately.
How to make sugar rimmed glasses:
Place ½ cup granulated sugar on a plate, moisten the rim of 4 glasses with a lemon wedge, turn each glass upside down and dip into the sugar until all sides of the rim are coated.