How to Make It
In a blender, puree all of the ingredients except the xanthan gum on high speed until smooth and bright green, about 30 seconds. Pour into a bowl. Slowly sprinkle the xanthan gum over while quickly whisking, until slightly thickened and foamy. Let rest for 15 minutes, then skim any foam. Pour 1 1/2 tablespoons of the mojito layer into eight 3-ounce popsicle molds or paper cups. Freeze until almost firm, 1 hour. Poke a popsicle stick into the center of each.
In another bowl, combine all of the ingredients except the xanthan gum. Slowly sprinkle the xanthan gum over while quickly whisking, until slightly thickened. Let rest, then skim.
Carefully add 1 1/2 tablespoons of the watermelon layer to the mojito layer in the molds. Freeze until almost firm, 1 hour. Pour the remaining mojito mixture on top and freeze until almost firm, 1 hour. Top with the remaining watermelon layer and freeze until firm, 2 hours. Unmold and serve.
The thickening agent xanthan gum is available at many supermarkets and on amazon.com. Simple Syrup: In a saucepan, simmer 4 ounces of water with 1/2 cup of sugar over moderate heat, stirring, until the sugar dissolves, about 3 minutes; let cool. Refrigerate for up to 1 month. Makes about 6 ounces.