Why Did My Coffee And Walnut Cake Sink?

Our answer

Nigella's Coffee And Walnut Layer Cake (from KITCHEN) has two layers of sponge that are sandwiched with a coffee buttercream. The sponge itself contains some ground walnuts, which helps to make the sponge moist and tender. As the sponge is tender you do need to be slightly more careful when you turn the sponges out of their pans and it may help to run a thin-bladed knife around the edge of the pan, then put a wire rack over the top of the pan and invert the pan and rack so that the cake turns out straight onto the wire rack.

From the description it does sound as if the cake was not quite fully baked when it was taken out of the oven. A regular oven setting of 180c is quite normal for sponge cakes, but as ovens vary slightly the cake could take a little longer to bake and the recipe does say "or until the sponge is risen and feels springy to the touch". The cake should be firm in the centre (feel springy) and you may also see the cake shrinking away from the sides of the cake pan. To be really sure, you can insert a cake tester into the centre of each sponge layer and it should come out clean. We would also mention that you should not open the oven door to check too early (allow at least 20 minutes of baking time) as opening the door can let gusts of cold air in and cause the cake to collapse if it is underbaked in the centre at that point.