Here is a link on instructions: Baking Cakes in Canning Jars.
In the oven right now are Nut Butter Blondies baked in jars! I prepare my jars a little differently than the link above. I do not grease my jars first. Rory eats them directly from the jar so it seems like a moot point to me. Also, baking times can vary quite a bit from original recipes, so keep an eye on them while baking.
The possibilities are endless! Have too many ripe bananas laying around? Make a batch of banana bread in jars and you'll have some 'fresh' banana bread to enjoy for weeks to come. Bored, but love to bake? Bake up a batch of your favorite cake. The single serving jars will help keep your will power in check! :) They also make great gifts and are suitable for fundraisers!