You open the oven to a cake that rose beautifully and is now a ring around a valley. Cakes sink because the structure sets too late, or never sets at all, and the air you beat in escapes before the crumb can hold it.
Six causes, and each leaves a slightly different mess.
1. The oven was too cool
The most common one. A cool oven lets the batter rise slowly and then collapse before the proteins and starches firm up. Most domestic ovens run below their dial; an oven thermometer costs very little and settles the argument.
Looks like: a wide, shallow dip and a pale, slightly damp crumb.
2. Too much raising agent
It sounds backwards, but more baking powder is not more lift. Excess gas creates bubbles too big to hold, they burst, and the cake falls. Extra soda also leaves a soapy taste.
Looks like: a dramatic rise, then a deep collapse, often with a coarse, holey crumb.
3. Overmixing after the flour went in
Beating butter and sugar for a long time is right. Beating the batter hard after the flour joins is not: it builds gluten, which makes a cake tough, and it knocks out air.
Looks like: a dense, slightly rubbery cake that peaked and sank modestly, with tunnels in the crumb.
4. The door got opened
Cold air on a batter that has not set yet drops the temperature exactly when the structure is most fragile. Leave it alone for at least three quarters of the baking time.
Looks like: a sudden crater, usually off-centre, on a cake that was fine when you looked.
5. The tin was the wrong size
A batter meant for two 20cm tins poured into one is too deep. The outside sets while the middle is still liquid, and the middle then sinks under its own weight.
Looks like: cooked, even overcooked edges with a wet, dense middle.
6. It was not baked through
The simplest cause. A cake can look done, brown and smelling right, and still be liquid at the centre. It sinks while cooling.
Test properly: a skewer in the middle should come out with a few dry crumbs, not wet batter. The top should spring back when pressed lightly, and the cake should have pulled very slightly from the sides of the tin.
The habits that prevent most of it
- Weigh everything. Cake is a ratio. Cups make the ratio a guess.
- Room-temperature ingredients. Cold eggs added to creamed butter curdle the mix, and a curdled batter holds less air. If it splits, add a spoon of the flour.
- Fold, do not beat, once the flour is in. Stop the moment you cannot see dry flour.
- Bake in the middle of the oven, one tray at a time if your oven has hot spots.
- Cool in the tin for ten minutes, then on a rack. Turning a hot cake straight out can tear it; leaving it in the tin entirely steams the base.
When it sinks anyway
Cut the collapsed middle out and call it a ring cake, or fill the dip with cream and fruit and call it deliberate. A sunken cake that tastes good is a trifle in waiting, which is the most useful thing anyone has ever said about baking.