How can I get my chocolate cake to taste less like a brownie?
In my experience, unsweetened baking chocolate can replace cocoa + oil at a ratio of 1 oz baking chocolate = 3 T cocoa powder + 1 T oil. I don't think this makes the result more brownie-like, but you do need to reduce the oil (or butter) in the cake to make up for the added oil from the baking chocolate.

If you use dark chocolate (or semi-sweet baking chocolate) reduce the sugar, too, but that's a little trickier to offer a hard-and-fast calculation.

If you are worried about losing the rich chocolate flavor by switching to cocoa powder, first make sure you're using a high quality cocoa powder and then you can also consider replacing some (maybe 2T or so?) of the liquid in your cake recipe with strong espresso. A small amount can enhance chocolate flavor in baked goods without actually tasting coffee-ish. 

Reducing the egg (or at least yolk) and oil in the cake should also make it less dense, but reducing too much can make it dry.