Heat a large cast iron skillet over medium-high heat. Add the vegetable oil and butter.
When the butter is melted and the pan is hot, add the corn kernels in an even layer. Let them cook undisturbed for 3-4 minutes until beginning to char on the bottom.
Stir the corn and continue cooking for another 3-4 minutes, stirring occasionally, until nicely charred in spots but still crisp-tender. Remove from heat.
In a large bowl, combine the mayonnaise, minced garlic, lime juice, chile powder, salt, and pepper. Whisk until smooth.
Add the charred corn to the dressing while still warm. Allow it to cool for about 5 minutes.
Gently fold in the cotija cheese, chopped cilantro, and sliced green onions.
Taste and adjust seasonings as needed, adding more lime juice, salt, or chile powder to taste.