I used an 85% chocolate for the Instant Chocolate Mousse and when the marshmallows started to melt, some oil 'leaked' out of the mixture. The oil wouldn't mix with the rest, so I didn't use it. My husband complained about a terrible smell coming from the kitchen, but the taste really doesn't reflect the smell. Perhaps I used the wrong chocolate?
Nigella's Instant Chocolate Mousse (from NIGELLA EXPRESS) is made with dark (bittersweet) chocolate as this gives a deep, rich chocolate flavour. Nigella suggests using a chocolate with 70% cocoa solids as this has a good flavour without being too bitter. A chocolate with 85% cocoa solids will be more intense but will also be more bitter in flavour. However it should still give a smooth mousse.
From the description given we suspect that the chocolate was heated too much and may have also burned a little. You need to use a low temperature and be a little patient while the ingredients gently melt together. If the chocolate is heated too much then the cocoa butter will separate out from the cocoa solids and we suspect that this is the oil that was observed. You may prefer to try next time with a chocolate with 70% cocoa solids as we believe it will give a better flavour, but we particularly suggest using a low heat and watching the saucepan carefully. It can also help to remove the saucepan from the heat while there are still a few small lumps of unmelted marshmallow and chocolate and then stirring until smooth. This usually prevents the mixture from becoming too hot and if the lumps remain unmelted then return the pan to the heat briefly and stir until just smooth, then remove from the heat immediately.