Thai Watermelon, Lime and Coconut Popsicles

1. Muddle the kaffir lime leaves and Thai bird chili with the sugar in a mortar and pestle until crushed and fragrant.
2. Remove the leaves and chilis and reserve the sugar for the popsicle base mixture.
3. In a small bowl, toss the reserved sugar with coconut and ginger. Add watermelon juice, lime juice and vanilla, and stir until the sugar dissolves.
4. Once the sugar has dissolved, add rum if using, and mix well.
5. Fill up the popsicle molds with the watermelon mixture and place into the freezer until completely frozen, preferably overnight.
6. To unmold, remove the popsicles from the freezer and let stand at room temperature for 5 to 10 minutes, or quickly dip in warm water.

Note: Alternatively, freeze the watermelon mixture in an ice cube tray and serve these colorful and flavorful ice cubes in a beverage such as agua fresca, limeade, or a martini.