Place 6 raspberries and 2 low-ball whisky glasses into the freezer to chill.
Add 50g caster sugar to a small saucepan with 25ml water and heat over a low heat until the sugar is dissolved. Bring to a simmer, then remove from the heat. Pour into a jug and leave to cool to room temperature.
Once the syrup is cool, pour 25ml into a cocktail shaker along with 50g raspberries. Muddle with a wooden spoon to release as much juice as possible from the raspberries.
Add 4 ice cubes, 100ml whisky and 40ml lemon juice, cover with the lid and shake for 15 secs.
Put an ice cube into each chilled glass. Strain the cocktail through a fine sieve into the glasses. Garnish with the frozen raspberries. There'll be enough syrup for more than this recipe, but it keeps, covered, in the fridge for a week.
