Chances are you may have noticed some new fixtures on the back bars of your favorite cocktail lounges. No, we’re not referring to lights or libations per se, but rather to wooden barrels—relatively small, 3-liter ones, as compared to the larger oaken casks normally used to store aging spirits and wines. These bar-sized barrels contain premixed cocktails that are developing increased depth, flavor, and finesse. And it’s all accomplished by the relatively simple process of premixing a large batch of cocktails an…