Put the mango cubes in the cocktail shaker and muddle until smooth.
Add mint leaves to the shaker and muddle gently to release the oils.
Pour in the rum, lime juice, and simple syrup.
Fill the shaker with ice and shake for 15-20 seconds.
Strain the mixture into a tall glass filled with fresh ice.
Top with club soda and stir gently.
Garnish with a mint sprig and mango slice.
Notes
The drink tastes best with ripe, sweet mangoes. Under-ripe mangoes can make the cocktail too tart.Store leftover mango puree in an airtight container for up to 2 days.Skip the rum to make a non-alcoholic version that's just as tasty.For a sweeter drink, add an extra 1/4 oz of simple syrup.