I don't know where I went wrong but I had a disaster with the Instant Chocolate Mousse. The mini marshmallows took so long to melt and when they finally did it was like the formed a big lump of goop and the water and butter just separated from the chocolate marshmallow goop.
Nigella's Instant Chocolate Mousse (from Nigella Express and on the Nigella website) is made with a base of melted mini marshmallows, butter, chocolate and hot water. These should be put in a saucepan and melted over a low heat, stirring the mixture occasionally. From the description it sounds to us as if the chocolate over heated, so the solids form a lump and the fats separate out.
Make sure that your saucepan is not too small as this could cause the mixture to take longer to heat up as well as making it more difficult to stir the ingredients as they melt together. You also need to keep the temperature low and if in doubt take the saucepan off the heat and stir and let it cool slightly. If you are still concerned then you could try melting the marshmallows, butter and water together and then lower the heat and stir in the chocolate until it has melted.