Combine sugar, mint leaves and half the lime slices (reserve the other half) in the bottom of a pitcher. Use a muddler or a wooden spoon to press the ingredients into the bottom and sides of the pitcher so that the mint leaves are crushed and the limes release their juice.
Pour the passion fruit puree, rum, syrup, ice and soda water over top and stir gently to combine.
When ready to serve, divide the mojitos between glasses and top with remaining half of the lime slices and some extra mint leaves.
This addition of fresh fruit puree is optional, but adds nice color and flavor to the drinks. Any tropical fruit will work. Passion fruit puree can be found in the freezer section even when the fruit is not in season. You can also use mango or pineapple; just cube the mango or pineapple and blend until smooth to get a puree.