Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the softened cream cheese, sour cream, and mayonnaise. Mix until smooth.
Add in the frozen corn, cheddar cheese, Parmesan cheese, chopped green onions (if using), garlic powder, onion powder, salt, and black pepper. Stir until all ingredients are well incorporated.
Transfer the corn mixture into a greased 9x13 inch baking dish, spreading it out evenly.
In a separate bowl, mix the crushed cornflakes with the melted butter. Sprinkle the buttered cornflakes evenly over the corn casserole.
Cover the baking dish with aluminum foil (if desired) to prevent over-browning and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and bake for an additional 15 minutes, or until the top is golden brown and bubbling.
Remove from oven and let it cool slightly before serving.
For a spicier version, consider adding some diced jalapeños or a sprinkle of cayenne pepper to the corn mixture.
This casserole can be made ahead of time and stored in the refrigerator before baking; just ensure to let it come to room temperature before placing it in the oven.
Leftovers can be stored in an airtight container in the refrigerator for up to 3 days. Reheat in the microwave or the oven until warmed through.