In a food processor, puree the watermelon flesh. Pour the pureed watermelon into empty ice cube trays and freeze for at least 4 hours. Also, freeze 4 martini glasses.
In a blender combine the frozen watermelon cubes, simple syrup, lemon juice, vodka, and melon liqueur; blend until smooth. Pour into 4 frozen martini glasses and garnish each with a lemon twist.
