I was living in Mexico City the first time I saw tlacoyos, and at first, I had no idea what they were. The thick masa patties—cooked on sidewalk grills in nearly every bustling Mexico City neighborhood—looked like little flattened footballs, puffing with steam on the hot comal almost as if they were breathing.To make them, vendors reached into buckets of freshly ground masa, pulled out hunks of dough, patted them into disks, and stuffed them with a smear of refried beans, salty requesón cheese, or fava beans, then grilled them until crispy. Just before serving, the women topped the tlacoyos with cooked cactus, raw cilantro and onion, crumbled cheese, and salsa. The whole process was mesmerizing. And they tasted as good as, or often better than, anything I'd eaten in high-end restaurants.Shortly after trying them, I became fixated on learning how to make them. Eventually I earned the trust of Rosa Peña Sotres, a street vendor and master tlacoyo maker who invited me out to her house about an hour and 20 minutes from Mexico City for a cooking lesson. Doña Rosa taught me how to shape them by hand. It’s a simple enough technique that a novice like me could make passable tlacoyos to start—but it takes years, if not decades, of experience to do it as quickly and well as Doña Rosa.If you have a Mexican grocery store near you, see if they carry fresh tortilla masa— masa made from nixtamalized, fresh ground corn—which makes the best tlacoyos. If none is available, masa harina (widely available at most supermarkets) makes a forgiving dough that crisps up nicely in a skillet or on a comal. If you don’t have a tortilla press, you can use a rolling pin to flatten and shape each tlacoyo, a tip I came across when researching my book Eat Mexico: Recipes from Mexico City's Streets, Markets and Fondas. Serve them hot, and don't hold back on your toppings—the final tumble of textures is what completes the dish.