My guidebook says
Sun Fat HK style cafe, on 26 to 28 Cameron Road in TST, is open 24 hours.
Macau Cafe on Jui Lok Road (not sure the English name) #25-27 is also 24 hours, and they have the signature pork chop bun and Portugese style baked egg tart.
Wing Wah is also nearby, not 24 hour, but they have an in house bakery.
Hankou Rd #43 is another HK cafe and on top of that they sell Cantonese BBQ (cha si etc).