Chill the cocktail glass in the freezer. Meanwhile, heat the sugar and 25ml of water in a small pan until the sugar dissolves to a syrupy consistency, swirling the pan occasionally.
Add the sugar syrup, lime juice and rum to a cocktail shaker. Lightly bash the mint leaves in a pestle and mortar, then add to the mixture with a couple of dashes of Angostura bitters and the ice cubes. Pop the lid on and give it a good shake, then sieve into the chilled glass. Top up with Prosecco and enjoy.