I don't understand why Jiffy doesn't include this recipe on their website. I remember looking for it there once before without luck.

This cornbread recipe is one that I once saw a friend prepare. She combined a Jiffy yellow cake mix with a package of Jiffy corn muffin mix, then added the ingredients (eggs, water, and/or milk) that would normally be added to each if preparing them separately. She used a 9"x13" cake pan to bake it at 350F. The time is the part I'm not sure of. I'd start with the recommended time on the cake mix, test, and bake longer if need be.

I hope this was of at least a little use to you.