This charming little amuse is comprised of two ingredients, and when there are only two, they had better be the best there are! My good[ friend Jose Andres inspired me to make this amuse one summer when he visited from his Washington, D.C., restaurant, Jaleo, where he is the chef-partner. Look for ripe, seedless watermelons in the height of summer, when they are bursting with luscious sweetness. Tap their shell; they should sound hollow and feel firm, never soft or mushy. Next, buy the oldest balsamic vinegar you can afford. You'll maybe have to purchase it at a gourmet shop, but once you discover the glories of 25-, 50- or even 100-year old balsamic vinegar, you will be hooked. This is not vinegar to sprinkle with abandon over lettuce or tomatoes, but instead a syrup elixir to use sparingly. Just a drop or two provides flavor that is rich, mellow, sweet and full — unlike any vinegar you have tasted before. When paired with watermelon, as I have here, it's heavenly.]