Can I substitue the dark chocolate with white chocolate in Nigella's Instant Chocolate Mousse?
We would not particularly recommend using white chocolate in place of the dark (bittersweet) chocolate in the Instant Chocolate Mousse (from Nigella Express and on the Nigella website). White chocolate doesn't actually contain any chocolate, it is a mixture of cocoa and other fats, sugar and vanilla. Unfortunately that means that it is less likely to set as well as the dark chocolate. Also as white chocolate is quite sweet it is likely to be too sweet when combined with the marshmallows.
However Nigella does have a couple of white chocolate mousses that you may like to try. In Express there is a White Chocolate And Mint Mousse that is very simple to make. There is also a White Chocolate And Passionfruit Mousse in Nigella Summer and on the Nigella website.