I have made Nigella's Madeira Cake twice and it has not worked on both attempts! I am a seasoned baker and can't figure out why the cake keeps collapsing completely.
Nigella's My Mother-In-Law's Madeira Cake (from HOW TO BE A DOMESTIC GODDESS) is a madeira cake baked in a loaf tin. The recipe is usually reliable and if the cake is sinking then it is most likely that the cake is not quite baked completely. With larger cakes it can sometimes help to rotate the pan during baking, for even cooking. We would suggest rotating the pan after about 40 minutes of baking. The cake should be firm on the top and a cake tester should come out clean when inserted into the centre. If the cake is browning on top too quickly then you can cover the tin loosely with foil towards the end of the baking time.
If the cake is coming out of the oven with a pronounced peak in the centre and then sinking then it is possible that the oven is a little too hot. This means that the outside edges of the cake cook quickly, forcing the softer centre inwards and upwards. The oven temperature in the recipe is for a regular oven so if you have a fan oven you may need to adjust it following the instructions in your oven handbook. As ovens vary slightly you may also prefer to try turning the oven down by 10c/25F and baking the cake for slightly longer.