I have not eaten any dim sum in KL, despite the years going up there & my Malaysian friends have not brought me to eat any either, I think, it is not their culture to eat tim dum. Cantonese Noodle House, I know one, it is at BB Plaza, the side of BB Plaza where the BB LRT station is. Not the front part, where they are constructing the MRT.
I think I've eaten at that Noodle house at BB Plaza. The wanton noodle is not bad.

In KL there are many interesting traditional chinese restaurants. It is run by someone with a title of "Chef". These restaurants are not usually located in the shopping malls, so unless you are a local you are unlikely to know about them. The ones you find at shopping centres are usually the new HK restaurants targetting the younger crowd. At the opposite side of Bukit Bintang, ground level near to the Green Elephant there's a HK restaurant of this type. You'll also find them at City Square Mall in JB.