I want to make my daughter a cake in the shape of a butterfly for her 3rd birthday and would really like to use Nigella's Buttermilk Birthday Cake recipe (Domestic Goddess, p210) and cut it to shape myself rather than use a butterfly shaped tin. I have had mixed results with this cake in the past and I suspect that the tin size may be the issue. I was planning to use two sandwich tins instead of the 23cm square tin but what diameter should these be? Alternatively if I wanted to use one round tin instead of a square one, again what diameter should this be? Finally, can you also please confirm what temperature and cooking time should be used for a fan oven? Any advice would be very much appreciated, thank you!
Normally to convert from a square cake pan to a round cake pan you would take the length of one side of the square pan and add 2.5cm (1 inch) to get the diameter of the round tin. However in this case the 23cm (9-inch) square tin is already quite large and a 25cm (10-inch) round pan is not particularly common, so we would suggest making one quantity of the cake mixture and baking it in a 23cm (9-inch) diameter round pan. Make sure the sides of the pan are at least 5cm (2 inches) deep. Fill the pan no more than 2/3rds full with batter (the leftover could be used to bake a couple of cupcakes).
Alternatively you could make up a double quantity of the mixture and bake it in a rectangular pan approximately 23 x 33 cm (9 x 13 inches). This would give you a good canvas from which to cut your cake.
The oven temperature is the same as for most cakes - 180c (350F, Gas Mk 4). The baking time for the 23cm round pan should be 30-35 minutes, but as ovens vary we suggest taking a peek at 25 minutes and testing the cake for doneness if the edges of the cake look like they are starting to shrink away from the sides of the pan. For a rectangular cake it should take 35-40 minutes but again it is worth quickly checking the cake after 30 minutes.