How to make chocolate fondant?
You can't really substitute marshmallows in that recipe. I have had some luck with the following procedure:


Get a good quality chocolate of the flavour you would like to use. Do not try chocolates with 70% cocoa solids or more. They are good for some things, but not for this.
Melt it in a double boiler. Do not make the chocolate boil or heat it directly (some can do that but most people end up with burned chocolate). Remove it from the heat.
Fold in 2 spoons of glycose syrup (for regular nonmilky chocolate) and mix fast. Let it cool and when it gets close to room temperature, put it in the fridge. For darker chocolates, use a bit more glucose and less for lighter chocolates.