i'v just recently made the swedish summer cake from Nigella's Kitchen book and whilst the actual cake sponge turned out lovely the custard filling did not mix well the cream at all. I found that the custard just did not seem to thicken up in the pan whilst heating it and was just like a pouring liquid. Please can you let me know why this would be and how thick the custard should be before adding the cream? Many Thanks
The photograph for the Swedish Summer Cake (on p267 of Kitchen) shows that the custard and whipped cream filling is fairly "loose" however if the custard is very thin it may not have been cooked quite enough. The recipe suggests a cooking time for the custard of 3 to 5 minutes, however if you are cautious with the heat under the pan, or if all of the ingredients are fridge-cold, then it may take slightly longer to cook. The best way to check if the custard is cooked is to lift the spoon from the saucepan and look at the custard coating the back of the spoon. Draw a line through the custard with a clean finger, if the custard flows back quickly into the line made by your finger then it needs further cooking but if the line is clearly visible for several seconds the the custard has thickened enough. If you have a digital or instant-read thermometer then the custard should start to thicken at around 80c (176F) but please do make sure that the mixture doesn't boil otherwise it will curdle.
The custard will thicken more as it cools and should be thoroughly cold before it is mixed with the whipped cream. If you have the time then it is a good idea to refrigerate it for a couple of hours (or up to overnight) before using. You can also speed up the custard cooling process by sitting the bowl of custard in a larger bowl filled with ice and stirring the custard over this ice bed until cool.
Finally if you are still worried about the consistency of the cooled custard then you can whip the cream for the filling slightly more firmly. once the full 500 mls (2 cups) of double (heavy) cream has been whipped to soft peaks transfer half to a separate bowl and whisk slightly more (being careful not to over-whip) before folding in the custard. Use the more softly whipped half of the cream for the top of the cake.