Add the mint leaves and simple syrup to a cocktail shaker and gently muddle to release the mint oils.
Add the aged rum, fresh lime juice, and Angostura bitters.
Fill the shaker with ice and shake vigorously for 10–12 seconds.
Double-strain through a fine mesh strainer into a chilled cocktail glass.
Top with Brut Champagne or Prosecco.
Garnish with a fresh mint sprig.
Double-straining is essential here - it removes all mint fragments and ice chips, ensuring a silky, clear pour that lets the Champagne bubbles rise cleanly. Audrey Saunders created this cocktail around 2001, and it has since become a modern classic.
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