How can I get my chocolate cake to taste less like a brownie? I've been testing out cake recipes, and I'm having a bit of difficulty creating a chocolate cake that doesn't taste like a brownie. 

What I want is a light, moist chocolate cake (doesn't everyone?), but the cake currently has the same texture and taste as a cakey brownie. I've tried aerating it more by beating the egg whites before I add them and adding more rising agent (tried baking powder, baking soda, and a combination of the two), but that resulted in the cake rising too much and overflowing the tin. 

I've also tried adding yogurt to make it more moist, but that only made it denser. 

I'm using real chocolate in my cake because I want it to taste super dark chocolatey. I've read a lot of cake recipes and I don't see very many that use block chocolate, usually only cocoa powder. Is this the cause of my problem?
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.