If I understand you want a non edible copy of a chocolate bar?
A silicone rubber mould should do fine for that.
Then you can cast it in whatever you want. If it's just going to sit on a shelf I would experiment with plaster, it's cheap and if you do the moulding right it will have a nice shiny coat, similar to a chocolate bar. You just need to get the colour right.
If you've never done moulding before there are plenty of resources online, just look for 'one part silicone mould' or 'silicone dump mould'