Pat the corn kernels dry, especially if they were frozen. Wet kernels steam and never brown.
Melt the butter in a wide skillet over high heat until it stops foaming.
Add the corn in a single layer and leave it undisturbed for 3 to 4 minutes, until the underside is browned in spots. Stir once and repeat, 8 to 10 minutes total.
Add the onion and serrano and cook 3 minutes more, until the onion softens.
Take the pan off the heat and let it cool for 2 minutes. Stirring dairy into a screaming hot pan splits it.
Stir in the mayonnaise, sour cream, lime juice and salt until every kernel is coated.
Fold in half the cotija and half the cilantro, then spoon into cups.
Top with the remaining cheese, the chili powder and the rest of the cilantro. Serve warm with a spoon and extra lime.