Open Club Sandwich
There is this place in Vienna called "Oben", which basically means "at the top". It actually is at the top of a building, namely the main building of the Viennese libraries.
It's a great place to hang out both during the day and at night and incidentally, they also serve good food. We went there last weekend and I had their Open Club Sandwich (don't ask me why they called it "open", because as you can see, it's not open).
Anyway, they used to have a similar one, but instead of classic rye toast they served it in a rather unwieldy piece of white bread. Not bad, mind you, but I've come to find that I prefer my sandwiches with less bread and more meat. And meat it does have. Some succulent chicken, taken right off a grilled chicken and then some bacon as well.
Also, some token vegetables, which I guess belong in there. They also serve some great home-made fries, which, admittedly, bear an uncanny resemblance to wedges, but I'm not complaining.
If you have the chance, do go there, but make sure to book a table, especially on Sundays, when they also serve a brunch buffet.
As a bonus: during the summer months, right behind the restaurant, they show films. So if you like to impress someone, bring them there, make them eat the sandwich, then enjoy a show.