Place blueberries and mint leaves in the cocktail shaker.
Muddle gently to release the blueberry juice and mint oils.
Add lime juice, simple syrup, and white rum to the shaker.
Fill the shaker with ice and shake for 10-15 seconds.
Strain the mixture into a champagne flute or tall glass filled with fresh ice.
Top with prosecco or sparkling wine.
Stir gently with a bar spoon to combine.
Garnish with fresh blueberries and a mint sprig.
Notes
Store fresh blueberries in the refrigerator for up to 1 week.Make simple syrup by mixing equal parts sugar and water in a saucepan. Heat until sugar dissolves, then cool before using.For a non-alcoholic version, replace rum with club soda and prosecco with sparkling apple juice.The cocktail can be made up to 2 hours ahead - just add the sparkling wine right before serving.