In a cocktail tin, combine first three ingredients with ice and shake. Strain into a large coupe, top with sparkling wine, and garnish with grapefruit peel.
In a pot over medium-high heat, slowly bring grapefruit juice to a boil. Add remaining ingredients and whisk until sugar has dissolved. Simmer until liquid is reduced by a third, then strain and cool.
From Pop, Fizz, Clink