The Nanaimo bar is a tremendously sweet, no-bake layered bar cookie from Canada made almost exclusively of packaged ingredients. The buttercream layer gets its yellow hue from custard powder, a vital ingredient that makes this a true Nanaimo bar. Use an offset spatula to give the chocolate top a surface as smooth as freshly Zambonied ice. The novelist Margaret Atwood included this recipe, from the chef Susan Mendelson, in her “Canlit Foodbook,” a fund-raising book that featured food from…