Prepare a grill to medium-high heat. Place the corn cobs on a rack about 4 inches above the fire, and allow to cook until it starts to char, around 5 minutes. Turn and continue charring until all sides are slightly blackened.
In a small bowl, stir together the mayo and lime juice, set aside.
Remove corn from grill, then rub each with about 1 teaspoon of butter. Take the lime mayo and rub each cob with roughly 2 teaspoons of the mixture. Sprinkle with salt, pepper, chili powder, then add the crumbled queso fresco. Serve immediately.