Walnut Acorn Cookies, tender walnut shortbread cookies dipped in chocolate and more nuts, got Gourmet magazine’s title “Best Cookie of the Year” in 2000. A favorite for Christmas cookie platters, they resemble acorns with their chocolate “cap.” We haven’t mastered the acorn shape, so ours come out oval-ish and a bit blobby. Even so, these relatively easy cookies look plenty fancy, taste rich, and make us feel like it’s a special occasion. And it turns out they’re vegan, too. We usually use only butter for shortbread cookies. But because walnuts make up a lot of the flavor here, we tried our favorite margarine, Imperial.* Its soft consistency makes it easy to blend and, in…