Remove the husks and silk from the corn but don’t break off the stalk end of the cob, so you have a handle. Mix mayonnaise, sour cream and cilantro together and reserve. Grill the corn on a hot grill or on a cast-iron grill pan until slightly charred on all sides.
While corn is still warm, slather each ear with the mayonnaise mixture, squeeze lime juice over the corn and cover with Parmesan. Dust with the chili powder, as desired, and serve with lime wedges.