IIRC you are either over cooking it or cooking it at too high a heat. the top is drying out and thus cracking. Try a lower temp and keep a better eye on it. probably got more advice coming up. hang in there.
"In those days spirits were brave, the stakes were high, men were real men, women were real women, and small furry creatures from Alpha Centauri were real small furry creatures from Alpha Centauri. "