Preheat gas grill to medium or prepare a charcoal grill. Grill corn, turning occasionally, until golden on all sides, about 5 minutes. When cool enough to handle, cut kernels from the cob; remove to a large bowl. Grill the chile and tomatillos, turning until skins are nicely charred, about 10 minutes. When cool enough to handle, peel the charred skin from the peppers with your fingers. Remove stem, core and seeds; chop chile and remove to the bowl. Finely chop tomatillos, capturing juices, and add to the bowl. Add avocado, onion, cilantro and salt. Coarsely mash avocado; gently stir to combine all ingredients.