Rencently when I bake birthday sponge cake the underside collapses leaving a hollow in my cake - what am I doing wrong?
maybe with more info we could help you a little more. what recipe did you use? do you have a photo of the cake?
I second this - what do you mean exactly the underside collapses?
With sponge cake, several things can happen:
You bake it too fast (too high temperature), the cake build a dome and if you check it / take it out early, the dome collapse.
To much temperature from top, the cake doesn't bake enough from the bottom and the middle. Usually the sides are OK, you take it out of the oven and the bottom middle is raw or not baked enough - it collapses from bottom.
Both has simple solution - lower the baking temperature and bake it longer.
Another thing can happen - do you make "classic" sponge - without baking powder, with separately whipped egg yellows and egg whites? Sometimes when mixing (folding) the batter, you could be over careful. Either the egg whites aren't mixed in enough, or there is still too much air in the mixture and it leaves hollow area in the cake. In this case, either take a knife and move through the batter or bang the pan on the counter before setting it in the oven. Don't be over enthusiastic - it needs only a little.
Could you post a picture what exactly has happended to you? Or even a recipe - not all sponge recipes are exactly the same, even if it should be the case.
If your cake is sinking in the middle, then it is under-done. Try baking longer at a slightly lower temperature. If you have an electric oven, put the shelf in the lower middle part, or your cake will brown too quickly on top making you think it's ready when it isn't (you should always test it with a skewer in the centre before taking out of the oven though, it should come out clean with no wet batter on it).