The “Beer Kir” at Marco Moreira’s 15 East is a Japanese beer-based mixed drink: Sapporo is floated on a shot of honeyed sweet potato vinegar, adding a sweet-sour edge to the dry lager. For the purposes of home experimentation, we found Benímosu ($11.35 for 4 oz.), the same artisanal vinegar used in Beer Kir at Katagiri, a Japanese food store on 59th Street. Made in Kyoto from purple potatoes and koji rice, Benímosu has clean,......