I filled the jars with alternating layers of chocolate cake crumbs, chocolate chips, and chocolate mousse. Here's my super-frugal, efficient tip for those cake crumbs. Whenever I make a cake that involves trimming it or say I make 2 cakes from a box mix and I only need one, I don't toss the extra bits - I throw them into a zipper bag in the freezer. This way, I always have cake on hand for dishes that don't need a full cake recipe, like a trifle or a parfait. Try it!
Now, regarding the mousse... I have made chocolate mousse from scratch before. The first time was a huge fail (because I grabbed half and half instead of heavy cream - oops!) and I ended up with chocolate soup. But now that I think about it, I recovered gracefully by heating up the soup and BAM - it became the best hot chocolate ever, so I'm changing that to a win. Life gives you lemonades, people.* My second go-round with mousse was a bit more successful, but required more time that I had last week, so I bought one of those box mousse mixes near the pudding and Jello mixes. It was actually very good. Oetker Dark Chocolate Truffle Mousse I recommend it if you are in a pinch.
*I have a friend whose husband always thought that the quote was "life gives you lemonades" instead of "when life gives you lemons, make lemonade" and I think that is the cutest, most uplifting story. I love positive people and I strive to be more like that.