With a hint of citrus and plenty of grown-up kick, this refreshing twist on the martini is perfectly formulated for hot summer nights. When buying watermelon, look for those that are symmetrical, with no flattened sides. The rind should be dull, not shiny, and just barely yield to pressure.
Place watermelon in blender. Cover; blend until smooth. Pour puree into a pitcher and keep very cold, even a little frozen to make it icy.
For two martinis, add one cup of watermelon puree, 1/4 cup of lemon vodka, two tablespoons of lime juice, one tablespoon each of Cointreau and sugar to the blender with three to six ice cubes. Cover; blend until slushy. Makes 6 large martinis.