Place the sugar and 1 cup water in a small pot, bring to a boil and remove from the heat. Place the lime leaves, mint sprigs and lime peel in the pot and allow to infuse for 30 minutes. Strain the lime and mint simple syrup and place in the fridge to cool. Place the plums and salt in a mortar and pestle and mash until roughly pureed. Divide the plum mixture, reserved mint leaves and ice cubes into 4 tall glasses. In a large measuring cup or pitcher, place the lime juice, sparkling water and lime and mint simple syrup. Mix well and pour into the glasses.