I came across this board from a google search for Dannon yogurt bars-they were covered in thin chocolate and were either vanilla or raspberry yogurt (like someone mentioned below). I remember getting them while growing up in suburban NJ. I don’t think they make them anymore…?
As for the "tang" of frozen yogurt that is hard to find now (since the advent of TCBY, etc.) I have the following suggestions:
Bloomingdale’s PLAIN yogurt (soft serve from a machine) still tastes the same from the 70s when I used to eat it with my mom during lunch at the mall.
Any plain yogurt from a real gelato vendor/store.
Finally, I have not tried it, but I heard that Pinkberry, a chain of fro-yo from L.A., which is now in NYC tastes like the original plain tangy fro yogurt…