I had a recipe for regular frosting at one time and all you had to do was add a bit of cocoa powder to it to make it chocolate. I can't remember it off the top of my head though, so here is one from All Recipes:
Now, I remember mine didn't call for evaporated milk and it also seems like I didn't use that much cocoa powder. I'd probably start with half that and then try it. Add more if it needs to be more chocolate-tasting.
If you don't have many ingredients on hand you could just make a simple chocolate ganache with just some chocolate chips (preferably dark chocolate) and some heavy cream. Just get a pot with a few inches of water simmering gently, and place a glass or metal bowl over it with the chocolate and a little cream poured in. Then gently stir it as it melts adding a little cream in at a time as needed. It will eventually melt down into a thick frosting like consistency. At this point you can spread it on the cupcakes or whatever. As it cools it will harden and firm up. The texture is basically like the inside of a truffle. Of course you can doctor it up even further with some extracts if you choose.