The first chocolate cake recipe I ever followed involved simply adding cocoa powder to standard sponge. I was scandalised and thought there must be a mistake, but was assured that this is how it's most commonly done. Years later, when I'd adopted the Tohellwiththat method of cooking, I just added a tonne of melted, high quality chocolate. No-brainer, really. It still rises perfectly, but it's much richer and not nearly as dry.
You don't, as a matter of fact, have to wait for a birthday to make this awesomely chocolatey, cream filled cake. A cake is its own reward. But for general health reasons, restricting intake to once a year might actually be quite advisable. Especially if you plan on eating the entire thing yourself. Which you will. I mean, if it's your birthday and you're making your own cake...
