Low Calorie Chocolate Idea

Well, I tried something the other day. And it didn't turn out quite as planned. Pretty simple but, I'm no chef so.
I used cocoa powder, sweet n low, and skim milk, in a sauce pan. Only warmed it up, should have heated more, and then poured it into a pan. Well I put it in the freezer to hopefully speed up the hardening process but, it never really hardened. It's more of a hard fudge.

Any ideas what I should do to actually make the chocolate harden like a chocolate bar?

Maybe heat it more, since I only warmed it?

I know this is crazy but, I love chocolate, and still being on my diet, I need to perfect the perfect homemade low cal chocolate =D

updated by @corey-lueneburg: 04/20/15 03:34:13PM

I don't think it will ever harden without cocoa butter - which has been removed from thecacao to give you your cocoa powder. And, if you added cocoa butter, you'd have to keep your ingredients in dried form - the liquid milk will cause it to seize and "harden" in a way you don't want. Good dark chocolate should only run you about 150 calories an ounce; you're better off adjusting your diet and eating that in moderation than trying to adjust chocolate to your diet.