Imhoff-Stollwerck Chocolate Museum, Cologne
Beware the shop on the way out! it's irresistable.
An interesting place, very modern inside and it even has a hot house that you can pass through and see the coco plants that are growing there.
Find everything you want to know about chocolate making process in this world famous chocolate museum. Near exit you can buy exotic chocolates (mostly chocolate noir) from all over the world.
Up the Rhein, you'll find the Schokoladenmuseum. As the name allready says, they show you the history and the making of chocolate.
Not bad, but you shouldn't go there hungry.... trust me!