Whisk mayonnaise, lime juice, and chili powder together in a bowl until evenly combined. Set aside.
Fire-roast until slightly charred. Or, heat a large cast iron skillet over medium-high heat and add corn and cook for 5 minutes without stirring. Toss around with a spatula and cook until corn has started to brown, about 3 minutes more. Remove from heat.
Add reserved mayonnaise mixture and stir until evenly combined. Add cilantro and stir again. Transfer to serving plates and sprinkle cotija cheese and green onion over the top.
Garnish with a lime wedge.
