If you want to play it safe, a neutral grain spirit, tasteless mistille, simple grape brandy or flavorless eau de vie does the job of raising a vermouth’s alcohol level. But we were never big on taking the safest path. We use small-batch apple brandy made by an artisan distiller in upstate New York. It’s not a simple way to up the alcohol content, but it is the reason Atsby vermouth has such a lush, jubilant flavor.