There are some places serving XLB but I'd say you will likely be disappointed, especially as you sounds like you know what a good XLB should be like.
Shanghai Garden at 220 N Congress Ave in Boyton Beach. Their Chinese menu is on a separate piece of paper. Not a lot of items and some they are out of. (or I suspect they just don't make it due to lack of demand) They do have XLB, Shanghai stir fried noodles and the like.
Asia#1 at 9409 Sheridan St in Cooper City advertises itself as authentic Northern noodle soup, with Dan dan noodle, Da Lo Mien, chicken cold noodle, flower bun, dumplings, etc, but oddly no mention og XLB. I've never been there myself though I have been wanting to for a while, just hardly made it there.
Hong Kong Dim Sum at 2363 N University Dr at Coral Springs is a Chinese take out place that sells frozen dim sum , and has a nice variety including XLB, shrimp dumpling, BBQ pork buns, wontons, rice paste, sticky rice chicken in lotus leaf etc. They can steam it for you to eat there, but it's a Chinese take out place with a couple of old and not very clean tables. I haven't tried their XLB but I think in general the quality of the ingredients is lacking.
Long Gung in Miami near FIU, on SW 8 St just west of Turnpike, also has some "Northern snacks". I kind of remember they have XLB but not 100% sure as it has been a while.
Just for the uninitiated XLB is a Shanghai dish, so the two requests would go hand in hand. From what I can tell, the Chinese in South Florida are largely Cantonese or Fujianese (many having passed through from New York or Cuba), so a Shanghai place would be unlikely. On the other hand I never expected a Taiwanese vegetarian restaurant would open up in Miami so anything's possible. Also as Frednesor's post alludes to, XLB occasionally shows up at Cantonese dim sum houses so there's always hope there.
I love XLB but they are very difficult to find here. There are a few places that used to offer them - Jumbo (now Hong Kong Noodles) in North Miami Beach; Mr. Chu (formerly on South Beach, now moved to Coral Gables and called Chu's). Both places took them off the menu before their reincarnations. Hong Kong Noodles does not have them, and I don't believe Chu's is doing dim sum anymore.
I have never seen them at Tropical, nor at South Garden, Kon Chau or Sang's on my less frequent visits there.
I have heard rumor that Philippe, just opened on South Beach, will be serving them, but haven't been there to verify.