How to substitute unsweetened baker's chocolate for powdered cocoa in cake recipe
It's just a really basic (plain-vanilla, if you will ;) chocolate cake recipe. You'd be better off just finding a basic chocolate cake recipe that calls for unsweetened chocolate in the first place, especially if you're new-ish to chocolate cake.

Otherwise the substitution that Sobachatina suggests above requires some fancy recipe-adjusting work on your part. Cocoa &amp; fat (melted butter, usually) can be substituted for whole chocolate, but it's not really such an exact science, so I don't think the reverse substitution will give you the results you're looking for.