Place a cane sugar cube into a chilled flute glass
Pour 0.25 oz of grenadine syrup, 0.75 oz of Campari and 0.75 oz of calvados into a shaker
Fill the shaker with ice cubes and shake
Strain into the flute glass
Carefully top up with champagne
Squeeze grapefruit zest over the flute glass and use it to garnish the cocktail