In a small food processor, combine the avocado mayonnaise, olive oil, cilantro, lime, serrano, garlic powder, onion powder and a few generous pinches of salt. Pulse until combined. Chill until ready to use.
Preheat the grill and clean the corn, pulling back the husks and removing the silks. Grill for about 8 minutes, rotating until char marks form on all sides. Remove from the grill and brush liberally with the sauce and remaining cilantro. Top with feta cheese and pinches of chile powder, if using. Serve with extra lime slices on the side.