Place corn perpendicular to grill or oven grates and grill or broil for about 10 minutes. Turn to evenly cook and char slightly. Remove and let cool slightly.
Cut kernels from corn cobs and place in a medium bowl.
Whisk together mayonnaise, butter, lime juice, cotija, cilantro, and salt in a small bowl. Add mixture to bowl with corn and stir.
Top with more cotija, cilantro, red onion, and a sprinkle of chili powder or Tajín.
Recipe Notes
Corn: Grilled corn is preferable, but you can use boiled or canned. For boiled, read my post on how to boil corn. For canned, use 4 15-ounce cans. Drain well and add corn and 2 cups water to a saucepan over medium heat. Let it simmer until the liquid is almost totally gone, about 5-10 minutes.
Mayonnaise: Use a good quality mayo without a strong taste. Sprout's Organic Mayo, Hellmann's, or Duke's are good options.