I'm not sure it would be easy to pipe it's a recipe that is spread hot like ganache. If you are after a piped recipe I am pretty sure I know the one and it involves mars bars melted into a choc icing and it sets thick and has that chewy fudge consistency.

If you google condensed milk fudge icing the ehow recipe is the one I know. I would copy and paste but can't from my phone.