Muddle cherries in a shaker tin until juicy. Add remaining ingredients, shake over ice. Strain into an ice-filled rocks glass. Garnish with a cherry with stem on.
Simmer sugar and water in a small saucepan over medium heat, stirring often, until sugar is dissolved. Remove from heat and let cool before using. Store syrup in an airtight container in the fridge for up to 2 weeks.