Verjus (from vert jus, or green juice), is the pressed juice of unripened grapes. When winemakers thin out their vines as crops begin to ripen, they can use those culled grapes to make this culinary treasure. Verjus is sour, sweet, and complex-tasting, like a softer, milder version of vinegar. In addition to adding depth to cocktails like this one it has a multitude of uses in the kitchen, from deglazing a pan to enrich a sauce, to subbing in for vinegar in salad dressings. It comes in red and white varieties. I've used white here, but either will work. Verjus is not fermented, and it's nonalcoholic.