In a small bowl, combine the mayo, lime juice, chili powder, paprika, and ¼ tsp of salt and mix well. Cover with plastic wrap and chill in the fridge.
Shuck the corn, leaving the end bit to act as a handle. Bring a large pot of salted water to a boil over high heat and par-boil the corn for 3 mins. Drain.
Heat a grill (or grill pan) over medium-high heat. Grill the corn, turning every minute or so using tongs, until nicely charred on all sides, about 6 to 8 mins.
Baste the grilled corn first with butter, then a second time with the chile-lime mayo. Sprinkle the feta cheese on top and garnish with fresh cilantro.