I always use unsweetened cocoa powder in my chocolate cakes. Melted solid chocolate is great for brownies, but I stopped using melted chocolate in my cakes within the first few months of baking because it's too heavy for cake. Every batch I baked with melted chocolate had a heavy, thick crust that was totally unappealing. Instead of adding melted unsweetened solid chocolate to the batter, I combine the cocoa powder with the dry ingredients. It gives me a great chocolate flavor and a rich but light, moist texture.
The commonly available unsweetened cocoa powder has 10% to 12% cocoa butter, which is alright but it also may be a little dry and dull. In my experience, there seems to be something in the chocolate liquor, the flavor-rich brown part of chocolate, that just doesn't bond with other fats as well as it does with cocoa butter. Bumping up the quantity of dairy butter isn't enough, adding cream makes the cake sticky, and vegetable oil merely coats everything in its path, making it feel moist when it's really just greasy. I've fiddled with chocolate cakes for years and still haven't found anything that works quite as well as unsweetened cocoa powder, with 22% to 24% cocoa butter.