Oh boy, 30 minutes for cupcakes? There is something wrong there for sure, even very wet batters with fruit take 23-25 minutes tops in a regular oven for standard size cupcakes… I think you have to purchase an oven thrermometer and test the temperature of your oven. Perhaps it is too low and you might need to adjust it? Or perhaps you are overmixing – I just made a batch this weekend using my homemade recipe that I created and have constant success with – well I beat the mixture (prior to adding flour) a little more than usual and my end result was sunkin cupcakes and I know it was becauase I over mixed it. I redid them, and beat my normal time, and they came out perfect. I agree with everything Chef Nicky says too.
Good luck and let us know how it goes on the next attempt!