Place sugar and 1/4 cup (60ml) water in a small saucepan over medium heat. Bring to a simmer and cook for 1-2 minutes until sugar dissolves. Remove from heat and cool, then cover and chill until needed.
Whiz half the berries in a food processor until smooth. Strain, discarding any solids.
Half-fill a cocktail shaker with ice. Add vodka, torn mint leaves, chilli flakes, raspberry puree and 2 tbs sugar syrup. Shake well, then stir in Prosecco.
Fill glasses with ice, strain over cocktail and serve with remaining mint and berries.