HK airport to guanzhou in the late hours

Upon your arrival, there is no direct bus from HKIA to Guangzhou, because the last direct bus is at around 22:15. There is no train from HK to GZ, either. So the best method is to order a cross-border bus on your own from travel from HKIA to GZ.
I heard the Trans-Island Limousine Service Limited Company provides such kind of service, so you can consult them.

I am not very clear your exact destination in HK, because there are several branches of Dorsett in HK, so it is hard to tell in detail. Generally speaking, you can walk to the Landmark Hotel in the Haizhu Square, where you can find direct buses for HK.