Most likely the dome is rising too fast and taking more than the usual amt of batter with it.
So this extra thick (heavier) dome has less structure to hold it up as it cools.
The hard texture is just the extra batter.
Try turning the temp down.
I agree, lessen amount of batter, lower your oven temp (our convection runs 25 degrees hotter than a conventional) and cook for a longer period of time until completely done.
In our oven I bake at 300 for about 20 mins.