I want to make a chocolate bar cake for my friends birthday, and am looking for a sweet/ icing idea to make something that looks like tin foil and is edible, any ideas?

I would think the closest thing to looking like tinfoil would be fondant that you paint with a silver lustre dust that you have mixed with a bit of vodka or lemon extract. I would colour the fondant with a bit of black to get it grey first and then paint. Or you could make a white chocolate candy clay and paint it.
Hugs Squirrelly

I totally agree with the Luster Dust! I just bought some today!!