I made the Easter Egg Nest Cake and the cake itself certainly looks just like the photo. However, when I added the still slightly warm chocolate to the lightly whipped cream it immediately went grainy and hardened. The result is "speckled" whipped cream rather than the homogeneously whipped chocolate cream I had hoped for. What happened?
Nigella's Easter Egg Nest Cake (from FEAST) is a sunken chocolate cake that is filled with a chocolate flavoured whipped cream. The cake itself sinks as it cools, giving a cracked, nest-like appearance and a perfect cavity for the cream filling.
The chocolate cream is made by folding melted chocoate into softly whipped cream and from the description it seems that the chocolate may have solidified quickly if the cream was very cold. This would cause the grainy texture as there would be specks of solidified chocolate in the whipped cream. If you are unsure then let the chocolate cool to room temperature and fold a couple of generous tablespoonfuls of the cream into the melted chocolate, then fold this mixture into the cream. Adding a little cream to the chocolate first will help to "temper" it, making it easier to fold the chocolate into the cream. Also make sure that no water gets into the chocolate while it is melting, particularly if you are using the saucepan of hot water method. Even the tiniest drops of water can make the chocolate sieze and turn grainy.