Without a doubt, conchas are the most iconic of Mexican pan dulce, a category that includes sweet rolls, pastries and cookies. They are such an important part of everyday life in Mexico that you're sure to run into one anywhere in the country, as long as there is a panadería or a small grocery nearby. The soft, brioche-style buns are firm enough to hold their shape while carrying a sweet and crumbly topping traditionally engraved with a seashell design. (Special concha molds can be ordered…