1. If you like you can use dark chocolate but make sure it is not very bitter. Use chocolate that has not more than 55% cocoa solids. 2. Do not worry if you are unable to get stiff peaks in the whipped cream, thanks to the marshmallows the mousse will set anyways. Just make sure that the cream you are using is not too thin (liquidy/watery). To achieve best results chill the bowl and the whisk in the fridge for about 1 hour before you begin whipping the cream. This helps especially if you are in a hot kitchen or live in humid climate. Cheat's version: If you have access to ready made (store bought) whipped cream you can use that instead of fresh cream as it will save you some effort. You can even substitute it with whipped cream powder like Dream Whip to reconstruct the cream - since I haven't used it I cannot tell you the proportions required - you may have to eyeball the quantity and add extra chocolate+coffee+marshmallow mixture if you end up using too much whipped cream. This is just a suggestion. Recipe adapted from: Coffee, 100 Everyday Recipes