My query refers to the Birthday Cake in Nigella's How to Eat. The consistency of the ganache when I make it is very thick and not pourable as described in the book and thus the finish of my cake is not flawless and shiny. Do you have any advice?
Chocolate ganache for icing cakes is generally an equal weight of chocolate and cream and it should set fairly well in a cool place. If your kitchen is warm you may find that you need to leave the ganache in a cooler room to thicken up. You can also refrigerate it briefly to help thicken the icing but you must be very vigilant as it will thicken very quickly around the edges - check and stir the ganche in the fridge every 3 to 4 minutes (it should only take 8 to 12 minutes to thicken if it has already cooled to room temperature).
The recipe in How To Eat suggests using a mixture of milk and dark chocolate however the milk chocolate does need to be fairly good quality - Nigella recommends Valrhona Lacte which has a cocoa content of 35%. This cocoa content is important as it is the cocoa solids which cause the ganache to set and a chocolate with less cocoa solids will have more fat and sugar and may not set as well. Green & Black's milk chocolate has 34% cocoa solids and you may find this easier to buy in UK supermarkets. Otherwise use all good-quality dark chocolate (70% cocoa solids) and sweeten the ganache slightly with 1 to 2 tablespoons of golden syrup or brown sugar, if necessary. We would also suggest trying the chocolate frosting from the Devil's Food Cake in Kitchen (p253) as an alternative (it is enough to fill and cover a 2-layer 20cm/8 inch cake), again you will need to let the frosting thicken in a cool place or the fridge (following the guidelines above) but it does give a wonderful glossy finish and remains slightly soft so is luscious to eat.