Line a 25 × 5 cm (10 × 3 in) cake tin with cling film.
Very slowly melt the chocolate in a bowl over a pan of simmering water. It is essential that the water does not touch the bowl and that the chocolate is not stirred. As soon as the chocolate becomes liquid, remove the bowl from the pan. Let the chocolate cool slightly, but do not allow to set.
Whisk the cream in a large bowl