For best results, use fresh, in-season sweet corn. If you don't have a cast iron skillet, a large stainless steel skillet will also work. Feel free to experiment with different types of cheese. Queso fresco or a mild cheddar can be substituted for cotija. For a smoky flavor, grill the corn on the cob before cutting off the kernels. This salad is best served fresh, but can be stored in an airtight container in the refrigerator for up to 2 days.