Arrange a rack in the middle of the oven and heat the oven to 350°F. Coat an 8x8-inch or other 2-quart baking dish with cooking spray.
Melt 8 tablespoons unsalted butter in the microwave in a large microwave-safe bowl. (Alternatively, melt on the stovetop and transfer to a large bowl.) Set aside until warm to the touch. Meanwhile, drain 1 (15-ounce) can corn kernels.
Add 8 ounces sour cream and 2 large eggs to the butter and whisk to combine. Add the corn kernels and 1 (15-ounce) can creamed corn and stir to combine. Add 1 box Jiffy mix and stir until well combined. Pour into the baking dish and smooth the top.
Bake until the casserole is puffed, slightly browned, and the center is firm, 45 to 50 minutes. Let cool for 10 minutes before serving.