Acorn, the second restaurant opened by Bryan Dayton and Steve Redzikowski, had a lot to live up to, considering it sprang from the mighty Oak at Fourteenth, their nationally recognized flagship in Boulder. And it wasted no time in doing so, appealing not just to Oak regulars who made the drive down U.S. 36, but also crowds of stylish American-food seekers flocking to the Source, which opened last summer on Brighton Boulevard. With just enough overlap between menus that the family line is never in doubt — recognize that kale salad, those fried pickles and braised meatballs? — Acorn has developed its own identity, focusing more on cocktails and approachable yet thoughtful small plates. The beauty of this approach is that rather than committing to one starter and an entree, diners can nibble their way around the globe, trying everything from lamb shawarma to squid-ink tagliarini and oak-smoked duck breast.