Making the grapefruit juice mixture: Bring the maple syrup and cinnamon stick to a light simmer and cook for 5 minutes. Take the maple-cinnamon mixture off the heat and let it cool for 5 minutes. Pour the cooled syrup into the grapefruit juice and refrigerate for 30 minutes.
Pour 1/2 cup of the grapefruit-cinnamon mixture and 1/2 cup prosecco into each champagne flute. Decorate as desired.