Preparation

Make the syrup

• Peel the rambutans. Using a paring knife and working over a bowl to collect juice, cut the fruit off the rambutan pits. In a small saucepan, combine the sugar with 1/2 cup water. Bring to a boil over medium-high heat, stirring occasionally, until the sugar dissolves. Remove from the heat and set aside to cool. Purée with the rambutans and juice in a blender until fairly smooth, about 1 minute. Refrigerate for up to 10 days.
• Combine 2 Tbs. of the syrup with the mint leaves in a highball glass. Squeeze in the lime, then add the wedge. Gently muddle to release the mint and lime oils. Add the rum, then fill the glass with crushed ice. Add sparkling water to taste. Stir gently with a straw. Peel and pit the rambutan, if using. Roll in sanding sugar, if you like. Garnish the glass with it, and serve.