These brownies must sit for some time before they reach their deeply fudgy peak. They are a perfect midweek recipe to yield a weekend's dessert, though in truth they'll be ready after only a night's rest. Once they've matured, you end up with a mystifying brownie that is firm to the bite yet melting in the mouth. Walnuts contribute crunch if you like that sort of thing. They are entirely optional. —Alex Witchel