For A double-recipe just double the instructions on the box to:
2 packages of Jiffy corn muffin mix.
2 eggs
2/3 cup of milk
Optional: 1/2 small apple cubed into small pieces
Stir the ingredients together and pour into a greased pan. In this case a greased glass baking dish selected so it would fit in the toaster over.
Preheat the oven to 400 degrees .
Bake for 25 min accounting the increased volume of cornbread mixture. You may want to cover the mixture with aluminum foil if you don’t like your corn bread with a too browned top.
I let my mixture in the toaster oven to cool down with the oven.