hagelslag (blowing hail?) are dutch chocolate sprinkles – invented in 1936 by mr. de vries, in reply to a child who wrote him asking for a chocolate topping to put on his bread. 
to use them, spread some butter on a piece of bread and sprinkle them on top. allow me to illustrate:
the butter is optional, but it helps them stick better. more than the different flavours, i think what makes hagelslag special is the texture – it’s just nice to bite the tiny crunchy pieces of chocolate, and let them slowly melt in your mouth…
they have a special kind of sprinkles called muisjes (little mice) that are spread on round rusks when a baby is born, and offered to friends or family. they’re either pink or blue, according to the sex of the newborn. isn’t that a nice tradition? :)