To make the syrup, combine all the ingredients in a saucepan over high heat and bring to a boil, reduce the temperature and simmer for about 8 to 10 until a light syrup forms. Remove from the heat and leave to cool completely before using.  Remove the cranberries once cold and pour the syrup into a sterile glass bottle.

To sugar the glass, place caster sugar on a plate. Run a piece of lemon around the rim of the flute glass then dip it into the sugar. Leave it to dry. 

To make the cocktail, pour in the cranberry syrup, followed by the vodka and ice and give it a stir. Top up with Prosecco,  add lime slices and a sprig of rosemary and serve.