Place jars in a preheated 325-degree oven and bake for 40 minutes. While cakes are baking, bring a saucepan of water to a boil and carefully add jar lids. Remove pan from heat and keep lids hot until ready to use. When the cakes have finished baking, remove jars from oven. Make sure jar rims are clean.
Place lids on jars, and screw rings on tightly.
Jars will seal as they cool. Cakes will slide right out when ready to serve. Unsealed jars should be stored in the refrigerator and eaten within 2 weeks. Sealed jars may be stored with other canned food or placed in a freezer. The cake is safe to eat as long as the jar remains vacuum-sealed and free from mold.