According to my Dutch friends, "boterhammen" just means "slices of bread", and isn't really a good way of saying "sandwiches" - does anyone else find they end up saying slightly misleading things?
A boterham can be one slice of a bread with a topping or two slices on top of each other, with stuff in between (cheese, meat, peanutbutter, traditional Dutch sweet bread toppings, like hagelslag, muisjes, etc.) like a "sandwich."
This is copied from the Van Dale online Dutch dictionary: "boterham 1(snee brood) slice (of: piece) of bread: (figuurlijk) iets op zijn boterham krijgen get sth. on one's plate; een boterham met ham a ham sandwich."
Edit: the Van Dale online Dutch dictionary translated sandwich to "dubbele boterham" (double sandwich), but to my knowledge that's not used much. One slice of bread is also oftentimes called "een sneetje brood." For example: Ik eet een sneetje brood met kaas = I'm eating a slice of bread with cheese.
The usage of dubbele boterham probably depends on the region, sounds quite normal to me (as are the examples of boterham in your post).
@DanielJohnOShea keep in mind that sandwiches outside of the Netherlands often have a lot more and also more diverse toppings. Dutch sandwiches usually only have butter and one ingredient on top or in between (traditionally butter with either cheese or ham). It's possible that your Dutch friends consider that a boterham and call something a sandwich (also in Dutch) when there are a lot of ingredients, e.g. mayonaise, bacon, cheese, lettuce and onion.
Isn't a sandwich with a lot of ingredients called a "een broodje...." in Dutch? For example "een broodje gezond."