Translation:The man is eating a sandwich with butter.
Do you mean that you need a second slice of bread to make a sandwich? Or that just bread and butter is not enough to make a sandwich because you need cheese or ham or something else?
If it's the first one, then a one-slice-of-bread sandwich is an obvious thing in Poland. If I'm eating at home, then I am unable to understand why someone would put a second slice of bread, it really doesn't make anything tastier and adds a lot of calories... if I make myself a sandwich to take to school/work, then of course the second slice is necessary or otherwise it would be difficult to take it ;)
If it's the second one, then I kinda agree, but "kanapka z masłem" still sounds okay to me. Also some people don't normally put butter on a sandwich... my brother absolutely hates butter :D
In British English, a sandwich with only one slice of bread is an "open sandwich". Sandwich means, "two pieces of bread with cheese, salad, or meat, usually cold, between them."
Kanapka was a useful word in Gdańsk.
So a "kanapka" could mean a single slice of bread with something, like butter, cheese, etc? For me (and I believe many others) to be a sandwich it needs two slices of bread. I am not trying to be one of those nosy guys, just merely trying to understand the Polish culture and language better :)
An 18th century English nobleman, the 4th Earl of Sandwich, is reputed to have ordered his servant to bring him meat between two slices of bread so that he wouldn't have to leave the gambling table while he ate. And a club sandwich is made of 3 slices of bread (Chicken and Lettuce Under Bacon).