Cinnamon Syrup: Fill a small pot with cinnamon, sugar, and water. Bring to a boil and cook, stirring frequently, until sugar is dissolved. Reduce heat to low and bring to a simmer. Cook, stirring occasionally, for 2 minutes, then remove from heat. Let sit for 2 hours.
Cinnamon Syrup: Strain cinnamon mixture into a clean glass jar or bottle.
Cocktail: Fill a tall glass with crushed ice and freeze until ready to use.
Cocktail: In a cocktail shaker, combine overproof rum, white rum, lime juice, Velvet Falernum, grapefruit juice, bitters, and 3/4 oz. cinnamon syrup. Fill shaker with ice, cover, and vigorously shake until outside of shaker is very frosty, about 20 seconds.
Cocktail: Add more crushed ice to prepared glass, if needed, to fill. Strain cocktail into glass.
Cocktail: Pour aged rum into a jigger. Slowly pour over cocktail.
Cocktail: Skewer grapefruit wedge, lime wedge, and cherry and garnish glass.