I tried the white chocolate mint mousse from Nigella's Express cookbook and it totally went wrong! It wasn't a mousse, but more like hard chocolate on top and liquid cream/water at the bottom of the shot glasses. First problem arose when I was putting a little bit of the softly whipped cream in the melted chocolate: it instantly got rather firm, with tiny lumps in it. And then when I gently folded the rest in it got sort of runny instead of creamy. I should also mention that my only aberration from the recipe was using cream with 32% fat (didn't find double cream anywhere!). So, please tell me, where did I go wrong? Is there a special kind of white chocolate one has to use or was it the cream? Thanks a lot for answering, I hope next time it will turn out perfectly as the flavor of white chocolate with mint is really great!
There are 2 potential sources of the problem of your mousse separating and not setting. The first is in the melting of the chocolate which could have become slightly over-heated while it was melting. Unfortunately white chocolate is a little unstable when melted as it has a high level of fats and no cocoa solids and it is very easy to "seize" the chocolate (turn it grainy and unusable) and also separate if you add other ingredients. When melting white chocolate make sure you keep a good eye on it, stirring regularly and remove the bowl chocolate from the pan of hot water while there are still some lumps of unmelted chocolate visible - keep stirring until the chocolate is smooth and this will reduce the risk of the chocolate overheating. If you are using a pyrex type bowl then these can also retain heat and you may want to put the bowl on a cool surface to help it to cool a little (but don't put it on a freezing cold surface as the glass can crack with a very sudden temperature change). Also let the chocolate cool sufficiently before adding the first spoonful of whipped cream.
The lower fat content of your cream could also be a problem. If you add small amounts of liquid to melted chocolate it can also seize the chocolate. However double cream has a high fat content (usually 45% or more for double cream or a minimum of 36% for heavy or whipping cream) and this fat content will help it to mix with the chocolate without causing it to seize. Using a cream with a fat content of only 32% means that it will have a higher water content (if it is whipping to soft peaks it is probably as it has some stabilizers or thickeners added) and this water content could be causing the choccolate to seize and the mousse to separate. So try to use a cream with a higher fat content.