Preheat your oven to 400°F (200°C). Grease and lightly flour an 8x8 inch baking pan.
In a medium mixing bowl, combine the Jiffy Corn Muffin Mix, egg, milk, and melted butter. Stir until just combined. Be careful not to overmix; a few lumps are perfectly fine.
Pour the batter evenly into the prepared baking pan.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown.
Let the cornbread cool in the pan for a few minutes before slicing and serving.