Oh the science of baking!
I too found that baking longer helped and agree with Little Cherry that chocolate is for some reason worse than vanilla, in fact I’ve NEVER had a vanilla peel on me (Hastily touching wood!). My favourite moist fluffy chocolate cake recipe just won’t work in cupcake cases so I have resorted to using the classic victoria sponge with cocoa powder which is still nice but not mega chocolatey!
Helen Ward, Amelie's Kitchen http://www.facebook.com/amelieskitchen