the cookies had to be cold before putting the choc. topping on
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When melting the chocolate for the topping next time, you could add a small amount off oil or butter- it'll still set, but you'll be able to cut it without the cracks... I always mean to do this but don't manage to remember very often!
Also,you could try turning the cookie upside down (so long as the top is flat...) and cutting through the biscuit base first you're not pressing down on the chocolate that way.