The best chocolate in Groningen (and, according to the shop lady, in all of the Netherlands, but she may be biased) can be found at
Bonbon Atelier Luca on Oude Kijk in 't Jatstraat 12. They pride themselves on using
only natural ingredients, and offer bonbons in such flavors as lemongrass, thyme & orange, walnut caramel, and passion fruit (my absolute favorite!)
For candy cravings, I recommend Droppie (A-straat 7), which offers a selection of traditional Dutch candy, beautifully displayed in old-fashioned apothecary jars. This may very well be the sweetest shop in town (although the shop girl could have done with a bit of sweetening up when we last visited).