Attach the cement board and plaster it with a smooth finish.

It would be best if you had a detailed sketch of the finished product so we could see where you're trying to go. No one sees/looks at the underside of counters so they don't need to be finished. But the side of the supporting "shelf" needs covering. You don't think the cantilever piece needs some kind of gusseting for support? Concrete is heavy and people will lean on it.

This is not mine, but it will look like this. I understand nobody looks at the underside, but would I want to leave the cement board uncovered? I assume I’ll use stucco on the underside to cover the cement board, but just thought I’d check to see how others have done it! The cantilever has hidden L braces attached to each stud for extra support, so it shouldn’t need any additional gusseting. I’ve ordered my frame modules from a respectable company who designs, builds, and focuses on outdoor kitchens. My design is very similar to many of the plans they sell, so I’m assuming it should be just fine. Thanks for your suggestions and help!

Functionally, it would work better without gusseting being in the way. But just make sure the L-brackets really are strong enough to support the cantilever. Underneath, I don't think it needs anything under other than framing. You don't want to block of anyone's access to where they can paste their bubblegum. :-) Just the edges need covering ... which, for me, would not be stone. It would be a contrast material since no one would ever actually build out of stone in a cantilever like that. It would look like it would break off in a heartbeat. You have to convince the viewer that it actually is strong. It shouldn't look like a material doing something that that material can't possibly do. In the top illustration the top material is wafer thin, which makes it look cheap. Let the top be a solid, beefy piece. It might have a surrounding edge that turns down.

Yes, my counter will be thicker than that in the drawing. I just showed that as an example. I'm still waiting for everything to be shipped, so I'll make sure the L-brackets are sturdy, before doing anything crazy. The frames will be 18 gauge galvanized steel, that customer reviews say they are very strong and sturdy. Also good reviews on the L-brackets. Guess, I'll find out. Thanks, for the input.