"Many Dutch people put hagelslag on their bread."
Translation:Veel Nederlanders doen hagelslag op hun brood.
It was always explained to me that "boterham" is both a sandwich, and also a slice of pre-sliced sandwich bread.
You can put slices of cheese and ham on a broodje, and have a broodje ham-kaas, but if the same ingredients are tussen twee boterhammen, then it's a ham-kaas boterham (though on many Anglified menus it's more often called a "sandwich"). Then, if you put it through a toaster, it's called a tosti ham-kaas, which cannot be made with anything but twee boterhamen.
To me it sounds like you put the package on top of the sandwich instead of sprinkling the chocolate on. (like Yruzz explained for "zetten"). Putting sprinkles on your sandwich is not such a deliberate(?) action to use *placing the sprinkles, it sounds like making art. You could use "strooien" instead of "doen". It would translate in "sprinkle sprinkles on their bread".
As other comments show, using "doen" here is hard for English speakers to comprehend. We put parmesan cheese on our spaghetti (not the container, but the cheese flakes). And we put a pinch of pepper in the sauce. This doesn't sound odd to us, as it apparently does to Dutch ears.
This sounds like a fixed phrase that has not been covered elsewhere. Are there other situations where to put something on something else requires the word "doen"? For example, would one "een tweede laag verf op zijn fiets doen"? If so, it's quite ideomatic, and I think this ought to be a section in itself, or added to the "plaatsen/liggen/zetten/stoppen" chapter.