Prep the corn by peeling the husks back (without detaching them from the bottom) and remove all of the silk. Replace the husks.
Place the corn on a grate over your campfire. Grill, turning frequently until the corn is charred in places and cooked through, 10-15 minutes. Remove from the grill and allow to cool a bit so they are safe to handle. Remove and discard the husks.
Combine the mayo, lime juice, chili powder, and salt in a small bowl. Slather evenly all over the corn. Sprinkle the Cotija and cilantro over the top.