All you need is a plain cake mix and a package of Jello.
Bake your cake per the instructions and let cool while you make the Jello, also per the instructions.
Then poke the cake, all over the top, with a straw. Most will tell you to use a pretzel or something solid like that, but a straw will pull plugs out of the cake which you can then squeeze out and eat! So, the straw is definitely the best way, in my opinion.
Then, instead of pouring the jello into a mold and making jello, pour it all over the top of your cake. It seem weird to pour liquid over a baked cake but trust me, just fill it up - then stick it in the fridge to allow the jello to soak in and set.
You can ice or frost it, obviously, but it's really best straight from the fridge, just as it is - without any icing. It just doesn't need it! And chocolate cake with lime jello is AWESOME! You get better stripes with a white cake, but who cares?!?! Call it a mud cake or an ugly cake but just try it - I promise, you won't be sorry!
You CAN eat it wet, without setting up, but you'll need a bowl. If there's any left, just stick it in the fridge and have it as it's meant to be, later.
Another awesome cake is chocolate cake with COFFEE jello - there are lots of recipes online for coffee jello/jelly. I don't have one of my own, but I've tried two different recipes - one made with brewed coffee and one with instant. I much prefer Colombian coffee so I liked the brewed better, but they were both pretty freaking good!