Slap mint hard on palm to release aromatics or on cutting board with the back of a spoon.
Add mint, blackberry syrup, lime juice and rum into a cocktail shaker and lightly muddle mint for 10 seconds to release oils.
Add ice to shaker halfway up.
Shake for 10-15 seconds.
Pour into a glass or mason jar.
Top up glass with ice and soda water.
Garnish with fresh mint and blackberries.