Old Fashioned Chocolate Cake Sinking

Our answer

Nigella's Old-Fashioned Chocolate Cake (from FEAST(books/feast)) is a chocolate sponge that is made with plain (all-purpose) flour plus baking powder and bicarbonate of soda (baking soda). The baking powder is less than one would normally use for a sponge as it is boosted by the addition of the bicarbonate of soda, which works well with the sour cream in the cake batter and usually gives helps to prevent cakes containing cocoa powder from being too dense.

From the comment we just wonder if self-raising flour is being used instead of plain flour. Self-raising flour already contains a reasonable amount of baking powder and if this is used plus the extra baking powder and bicarbonate of soda then this can cause cakes to rise up a lot in the oven then sink back on cooling. The extra leavening means that there are a lot of carbon dioxide bubbles in the batter and these expand in the heat of the oven. The excess bubbles expand a lot in the heat of the oven and stretch the gluten strands in the flour too much. The gluten strands then become weak and break, rather than setting as the cake bakes. So when the cake comes out of the oven the structure cannot be maintained and the cake sinks back as the bubbles cool and contract, even if the cake is baked fully. You usually see a very wrinkled surface to the cake if too much leavening is used. If plain flour is being used then make sure that the baking powder and bicarbonate of soda are measured carefully, using 5ml and 2.5ml measuring spoons and using level spoonfuls.