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.