I made the Chocolate Guinness Cake for my brother's birthday in August and it was delightful! Last time, the cake sank a little in the center, but nothing that a nice layer of frosting couldn't fix. I just made another one tonight and took it out of the oven about an hour and a half ago. But this time, it sank! I didn't change anything and followed the recipe as stated. What could have caused the cake to sink so much? Maybe I moved it around a little too much while I was checking it when it was baking? Thank you for your time and expertise!
Nigella's Chocolate Guinness Cake (from Feast and on the Nigella website) is a perennial favourite and the recipe is one that most people find quite reliable. Generally if a cake sinks in the centre it is not quite cooked properly. It is possible that if you checked the cake too early then a gust of cold air from the opening and closing of the oven door could have caused the undercooked centre to sink back.
If you are checking the cake then let it bake for the minimum 45 minutes stated in the recipe. Carefully open the door a little and gently touch the top of the cake (being careful not to touch the pan and any of the oven or oven shelf with your arms or hands). If the cake feels firm on top then you can check with a skewer. Open the oven door slowly and try to insert the skewer into the cake without removing the cake from the oven or moving it too quickly. However if the cake feels very soft on top then let it bake for another 5 minutes and check again.
As the cake is made with bicarbonate of soda (baking soda) then it is possible that this was the cause. Make sure you measure the bicarbonate of soda carefully as too much can cause the cake to rise up and then sink back dramatically. Also make sure the oven is preheated before you make the cake batter so that the cake can go straight into the oven once mixed. If the batter is left standing for too long then the bicarboanate of soada will become less active and could lead to a sunken cake.