Great question — answering as the UphillTech team:
I have never tried cooking a partially-done cake in a microwave to finish, but I don't expect it to turn out a desirable result. You may be out of luck for the current batch. In future, I recommend going by internal temperature, rather than total cooking time. Depending on the recipe, cakes using wheat flour tend to set around 200F (93C), so aim for 205F; you may need to do a batch or two as an experiment before you find out the perfect "done" temperature for your cake. Get yourself an instant-read thermometer or probe thermometer; make sure the tip of the probe is in the very middle of your cake when you take your readings. Also, keep in mind that the cooking process continues even after...
Full answer by Bruce Goldstein: https://cooking.stackexchange.com/a/19441 (CC BY-SA 3.0).
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