These savory stuffed corn cakes have a long history in El Salvador and Honduras. But for a version that even a rookie could pull off, we had to bring in some new techniques. Hydrating the masa harina with boiling rather than room-temperature water allowed the starches in the flour to absorb it more quickly and completely, resulting in a well-hydrated dough that was easy to work with and didn't dry out when cooked. The proper ratio of masa dough to filling ensured that each bite of pupusa included plenty of melted cheese filling. Pressing the stuffed pupusas into 4-inch disks between sheets of plastic ensured uniform thickness and allowed us to cook four pupusas at once in a 12-inch skillet. The crunch and acidic brightness of curtido and salsa perfectly complemented the tender, savory patties.