Since I first recieved my sourdough cake starter "Herman", I have been enjoying experimenting with using it to make cakes, pancakes and biscuits (cookies). I can adapt just about any recipe to Herman, and he adds a slight sour tang and creates a light fluffy batter. Here's what I do to adapt a recipe, just use all the same quantities, but combine them in a different order:
Melt the butter in a pot over a low heat. Stir in the flour, rapadura (sugar), and flavours (cocoa, ginger, spices), and add a slurp of Herman and any other liquids, put on the lid and leave the mixture at room temperature for a few hours (you can start this in the morning and bake in the afternoon, or even put it all in the fridge after a few hours and bake it later the next day, just depends on the outside temperature, in winter you could probably leave it out overnight).
When you're ready to bake, add the egg and a good heaped tablespoon of baking soda (leave out the baking powder). The baking soda is enough to rise the batter because the fermentation has made the batter acidic and the baking soda is alkaline, when the two mix, they produce carbon dioxide (and water and a salt) which rises the batter (baking powder contains both baking soda and citric acid, and works in the same way, if you add baking powder to water you will see it bubble as the carbon dioxide is released).
Herman is very easy to care for. I keep Herman in a jar in the fridge, where he lasts for weeks between baking. Every few weeks I either use some Herman to bake something or I just tip about half of it down the sink (boosting the microbes in our septic tank), and top up the jar with a few spoonfulls of flour, rapadura and some milk. I give it a good stir, leave it out at room temperature for a day and then put it back in the fridge.
Apple and berry cake
Actually the other week I left him a bit too long and he had a nice layer of blue mould growing on top. I thought maybe I'd finally killed him, but I decided to try to revive him, so I scraped off the mould, added flour, rapadura and milk and left the jar on the bench, and a few hours later Herman was bubbling away like his old self. I was intending to throw out the first batch and top him up again, but it smelt fine, so I went ahead and baked a cake instead. You just have to trust your nose with fermentations!
The advantage of fermenting with Herman before baking is that some of the protein and carbohydrate in the flour and sugar is pre-digested, like with sourdough bread, so its easier to digest. Also I swear it produces a fluffier cake, without any "creaming" (I don't have a cake mixer, but I suppose its good arm exercise if you have to do it). And its kind of fun to ferment.
If you want your own Herman, you can try to find someone who will give you some of their's, or you can try to make your own (its not ideal, because is based on commercial yeast rather than wild yeast, but I'm sure you get a bit of everything in there after a while!).