Method

• 1.
  Using a small ice cream scoop or melon baller, scoop out 16 balls from one half of the watermelon. Place on a tray and freeze for 1 hour.
• 2.
  Chill 4 generous glass tumblers. Roughly chop the remaining watermelon, discarding the rind, and place in a blender with the vodka, Cointreau and ice cubes. Whiz until the mixture is smooth and slushy - you'll probably have to do it in batches.
• 3.
  Arrange the watermelon balls in the chilled tumblers, and insert 2 basil leaves down the side of each glass.
• 4.
  Strain the cocktail mixture through a sieve, pour it over the watermelon balls, then serve.