Preheat your grill to medium-high heat. Place the corn cobs on a rack about 4 inches above the fire. Cook until the corn starts to char, about 5 minutes. Turn each ear 90 degrees and continue charring until all sides are slightly blackened.
In a small bowl, stir together the mayo and lime juice. Set aside.
Remove the corn from the grill. Rub each ear with about 1 teaspoon of butter and then with 2 teaspoons of lime mayo.
Sprinkle the corn all over with salt, pepper, and chili powder. Then sprinkle with crumbled queso fresco and chopped cilantro, if using. Serve immediately.
Notes
If you can't find queso fresco at your local store, you can substitute cojita cheese instead.