This bubbly cocktail begs to be eaten with a sweet, spiced cookie.
In a large pitcher or bowl, combine sugar, maple syrup, and sparkling water until sugar is dissolved.
Stir in gin, sweet vermouth, lemon juice, and grapefruit juice and refrigerate until chilled, approx. 1 hr. Meanwhile, slice lemon and grapefruit into wheels for garnish; set aside.
Just before serving, add sparkling wine and float grapefruit and lemon wheels on top to garnish.
Dust cinnamon over punch. Serve on ice. Enjoy!