I am usually able to knock up a batch of fairy cakes or cupcakes quite easily, however have just had two baking disasters in a row. Firstly I tried to make some of the basic Cupcakes and though they cooked nicely in the oven, they went flat and very rubbery upon cooling. I then tried to use the Old Fashioned Chocolate Cake recipe to make cupcakes, something I have done successfully several times before, however, though these rose splendidly, upon taking from the oven, they too went completely concave. They also rose in a funny shape. Is it likely that my oven is off, or perhaps that the new paper cases I have brought are the wrong size?
Nigella's Cupcakes recipe (from HOW TO BE A DOMESTIC GODDESS) and the Old Fashioned Chocolate Cake (from FEAST) are slightly different styles of sponge cake and both recipes usually rise very well. However from the description we suspect that the oven you are using may be running a little too hot.
When an oven is too hot sponge cakes rise very quickly and often have a sharp peak in the centre. This happens because the outside of the cake (or cupcake) bakes very quickly, pushing the centre inwards and then upwards. The top will also brown quite quickly. But the centre may not be quite fully cooked so when the cakes come out of the oven and cool then the centres collapse and sink. If you have a fan oven then you may need to adjust the temperature downwards, following the instructions in your oven handbook. If you have a conventional oven then it may be worth checking the oven temperature with an oven thermometer. These are relatively inexpensive and can be bought at many cookware stores. If this shows that the oven is too hot then the thermostat may need adjusting. Also make sure that you measure the baking powder and/or bicarbonate of soda (baking soda) carefully as too much raising agent will also cause the cakes to rise a lot in the oven and then sink back on cooling.