I would purchase a small amount of chocolate malt.
Steep this in hot water to extract the fermentables, boil it to sterilise it and then add it back to the wort you have produced from the kit. Then just ferment out the wort as per the kit instructions.
You may have to play around a wee bit with the amount of chocolate malt you use to get your desired flavour profile.
I think I also read a post somewhere about someone adding a cup of strong coffee to the wort prior to fermentaion - check that one a bit further though as I am not 100% on that.
ATB