In a medium bowl, combine corn, sour cream, lime juice, chili powder, cotija cheese, and a pinch of salt. Stir to combine. Set aside.

In a separate large bowl, combine mashed avocado, red onion, garlic, tomato, ½ teaspoon salt, and a pinch of black pepper. Once that's combined, stir in a little more than half of the corn mixture. Taste and add more salt if necessary. Transfer to a serving bowl. Spoon the remaining corn mixture on top of the guacamole and sprinkle with additional cotija cheese, chili powder, and/or chopped cilantro. Serve immediately or refrigerate tightly covered for an hour or two (any longer and it might begin to turn brown).