When I first looked into the ins and outs of bottling fruit and vegetables, I came across the somewhat unusual concept of bottling cakes, which was described in a German canning book called "Weck's Einkochbuch." (Weck is Germany's oldest bottling company.)
I know this sounds bizarre, but it is possible! Obviously, this is not as essential as preserving surplus food from the garden. Nonetheless, I did give it a go just for the fun of it. It is actually kind of handy to open a jar of cake to serve to unexpected visitors, and it allows you to bake several cakes in one sitting and keep them fresh for up to six months without the need for a freezer.
However, you do need to make sure that the jars you are using are either perfectly straight or better still, conically shaped with the wider end at the top, otherwise you will be serving a jar of crumbs (in which case your visitors will think you are not just slightly unusual but downright mad).
I used straight Fowler's Vacola jars (as seen in the picture). However, as the quality of the glass used in Fowler's jars varies substantially, you need to check carefully for little knobs or uneven surfaces on the inside of the jars, otherwise it becomes close to impossible to get the cakes out. The instructions below assume you are using a system similar to Fowler's or Weck's, consisting of jars, rubber rings, lids and clips. If you are using a different system (eg Mason jars), adjust accordingly.
This is how to do it:
Choose a recipe you like (any mix using baking powder as rising agent should do).
Fill clean glas canning jars half-full with the cake mix, making sure that the bottle rim remains perfectly clean so you won't have crumbs stuck to it later on.
Bake your cakes at moderate heat (160-180 degree Celsius) in the oven for 60 minutes or according to your recipe. Check with a wooden skewer whether the cakes are done.
Remove cakes in jars from the oven. If a cake rose above the top level of its jar, cut the excess off with a sharp knife.
Put on the bottling ring and lid, attach the clips, put jars in a hotwater canner, fill with water up to 2/3 the height of the jars and process at 100 degrees Celsius. It is important that the temperature of the water you add to the canner is the same as the temperature of your jars, otherwise you risk breakage.
Processing time is 20 minutes if the jars were still quite hot when you put them in the canner. It is 30 minutes if you allowed the jars to cool down.