How to Make It
In a blender or food processor, puree the raspberries with the sugar and 4 ounces cold water. Transfer the raspberry syrup to a jar, cover and refrigerate overnight.
Strain the syrup into a pitcher, then stir in the bourbon, grapefruit juice, lime juice and bitters. Refrigerate until chilled, about 2 hours.
Stir the drink, then pour into 8 chilled ice-filled collins glasses. Stir 2 ounces of club soda into each drink and garnish with the grapefruit twists.