This recipes was adapted from one published in the Border Grill Restaurant newsletter. The original recipe called for baking the yams (actually sweet potatoes called garnet yams) for one hour, and uses flour tortillas. Either flour or corn tortillas will work, I prefer the corn which are used more in Central Mexico where I developed a taste for Mexican food. The cotija cheese does not melt very much, it's a dry cheese intended as a garnish. Here in California, Cacique is the popular brand. If cotija is not available to you ,any dry, crumbly, non melting cheese will substitute. The amount of yam mixture may be more than is needed for 4 quesadillas (depending on how much you pile on) however, the leftovers make a delicious side dish. I like to make some extra as planned-overs.