Did you check the oven temp--using better chocolate may
be best, but I think Charlie has it. I've used chocolate chips in things like this and it is a comparatively small amount, in my opinion.
A supporting quote
temperature inside your oven can differ from the temperature indicated on the oven dial. All ovens need recalibrating occasionally. If the oven temperature is too low, the structure of the cake may not set sufficiently,
And mixing correctly is important also
How about the freshness of your baking powder?
Your post seems to indicate that it always happens when you make it. Is that correct?