Recently I've been baking cakes, well one cake to be precise - a banana and lemon drizzle cake - a couple of times. Both times it has been slightly different and thanks to this article in the New York Times, I now know why.
Turns out it's the butter.
The gist of the article is that butter is like concrete. It holds everything together. If it gets too hot it breaks down. Being lazy, I melt my butter by zapping it in the microwave. NO!!!! I now know that this ruins it, the bonds will never return again. The butter should look firm, feel cold to the touch but will leave the impression of your finger if you touch it. Then you're ready to cream it and make cookies/cake/brownies etc. It's so important because creaming the butter is what creates the air bubbles in cakes to rise. Baking soda only expands air bubbles, it can not create them.
So there you go - look after your butter.