Heat vegetable oil in a large skillet until shimmering. Add corn, season with salt to taste and toss, then let cook, tossing only occasionally (about every 2 minutes) until corn is well charred all over (about 6 - 10 minutes total). Remove from heat and allow to cool slightly.Add corn to a medium bowl along with, onions, cilantro, jalapeno, garlic, 3/4 of the Cotija, mayonnaise, 1 Tbsp lime juice and chili powder and toss. Pour remaining 1/2 Tbsp lime juice over diced avocados then add to bowl with corn mixture and gently toss. Top with remaining Cotija and additional chili powder to taste.Recipe source: adapted from Serious Eats