First thing, I want to put a disclaimer out there. For years people used this method to "can" cakes. These really aren't safe to put on your shelves for years as food storage. There... got that out of the way :)
These are a super fun and easy gift idea. Make sure you use wide mouth jars. I used pint jars, sprayed the inside with cooking spray and filled the batter (from a cake mix) to the 1 cup or halfway mark. I baked them at 350 for 30 minutes and topped them with new sterilized lids and rings. The lid will actually seal which keeps the cake nice and fresh while it travels through the mail in a care package. Once you open the jar the cake slides out perfectly and can be sliced and frosted. I tried one out and they came out perfectly.
What an awesome way to send baked goods! Wrap carefully and very well in bubble wrap when mailing. Don't forget the frosting and knife. I added in paper plates and napkins as well.