Muddle the lychees, lychee syrup, lemon juice and rosewater in the cocktails shaker using the end of a clean rolling pin.
Add the gin and a handful of ice cubes and shake for 30 seconds.
Strain into two chilled Champagne flutes (or coupe glasses) and top up with the Prosecco – be careful as the wine has a tendency to froth over the glass.
Decorate with a small slice of lychee and serve immediately.