Prepare the cake according to package instructions, or use any cake recipe.
In pint size, straight-sided wide-mouth jars, put 1 cup of batter in each greased jar.
Make sure to keep the rims of the jars clean.
Put in preheated oven 350 degrees F. Place jars on a cookie sheet to keep from tipping over while baking. Bake for 30 to 35 minutes, or until a toothpick inserted into cake comes out clean.
While they are baking, have your jar lids boiling in a pan of water. When the cake is done, take one jar out at a time and add the hot lid and screw on your jar ring and let set and cool.
It will seal as it cools. Place the jars on the counter and listen for them to 'ping' as they seal. If you miss the 'ping', wait until they are completely cool and press on the top of the lid. If it doesn't move at all, it's sealed.
After it cools it will pull away from the jar and when you are ready to eat, open and pop out the cake and enjoy.
Unsealed jars should be stored in the refrigerator and eaten within 2 weeks. Sealed jars may be stored for up to 3 months.