The sopes resemble dainty hors d’oeuvres. Round, with pinched edges that leave the tiniest indented center, these corn masa preparations are about a bite-and-a-half big and come four to an order. From their middles rise layers of smooth refried beans, peaks of chopped protein (I chose chicken), dollops of guacamole, slivers of pickled red onions, and sprinkles of queso fresco. The sopes could be described as cute. But that would be a disservice to their construction and history.These fantastic morsels are the sleeper hit at Jaime Tovar’s Mission X Tacos, a trailer stationed at the Hill Country Ice House, in far southwestern Austin. The location is a slog from the city’s bar and taco-truck hubs, but it’s a respite from the clogged traffic and crowded…