Fill a double old fashioned glass with crushed ice to the top. Add all the ingredients to a cocktail shaker with ice and shake for 15 seconds. Strain into the glass and garnish with sliced lime and mint springs. Optional: float 1/4 oz of rum on top.
Notes
Fresh passionfruit juice is great for this, but I buy a frozen puree that has very little sugar added to it. This is important because the drink is supposed to be a bit tart.