Add the mint leaves to a tall glass along with the rum, lime juice and sugar syrup and gently muddle to release minty flavour.
Fill the glass with crushed ice and gently stir to mix the syrup as evenly as you can. Top with sparking wine and gently stir again. Serve with a straw.
Notes
For a less alcoholic version of this cocktail, use soda water instead of sparkling wine.