chocolate vs cocoa cake?

Hello,

My friend and I are going to make a chocolate cake for her little sister, I'm quite used to baking, however, one of the things I have often asked myself is whether chocolate cake is better made mostly of cocoa powder, or chocolate (as in solid).

I have only made one with mostly cocoa, and I think the ones I have made with solid chocolate have tasted more richer and denser, what do you think?

Chocolate vs Cocoa?