PREHEAT oven to 350 degrees.
SPRAY a 9 x 13 baking dish with cooking spray.
In a medium skillet, MELT butter.
SAUTÉ onion until onion is opaque.
In a large bowl, COMBINE sauteed onion, corn, cheese, chilies (if using), garlic powder (if using), eggs, and Jiffy Corn Muffin Mix . STIR to combine.
TRANSFER mixture the 9 x 13 dish.
PLACE dollops of sour cream evenly spaced over the mixture.
Gently SWIRL in the sour cream into the mixture.
BAKE for 45-50 minutes, uncovered, until lightly browned and center is still a little soft.