Wash your mint leaves and place them in the cocktail shaker
Add simple syrup, and lime juice, and gently muddle to release the oils and flavors from the leaves. (If you over-muddle your cocktail can become bitter).
Add white rum, a handful of ice, and a splash of soda water, and stir the mixture with a bar spoon.
Now cover and shake the cocktail vigorously for about 10 seconds. This not only blends the ingredients but chills the cocktail too.
Pour the mixture into a glass with more ice and top with the rest of the soda water.
Garnish with an extra sprig of mint and a lime wedge.