I knew two Chinese language centre in CUHK and HKU providing Chinese language course in HK. They both have established over 30 years.
I come across with the staff and the teaching materials from both centre, therefore, I can recommend them here. If you live in Hong Kong, and want to pursue a full time [or part time] Cantonese or Chinese study, this maybe a great choice for you. Needless to say, they have strong academic backgrounds and strong teaching fellows. Both centre provide certificate or diploma course.
Their website are below.
CUHK = Chinese University of Hong Kong [It use the yale romanization]
HKU = University of Hong Kong [It use the jyputping romanization]
There are other private language training centre with different locations, catering individuals’ preferences.