It's probably a mis-translation from German. "Knack" is a german word that describes a specific sound - I'm not sure there is a precise English translation. The dictionary offers "click" or "snap" as a translation, "crack" would be another similar word.
I assume that in this case "Knack" refers to the bite of the chocolate and describes a hard/brittle chocolate.
One question - I noticed one of the tasters said “Knack.” twice in the comments. As an American, I’m not familiar with that word being used this way. Is this just a way of saying “Clever” or something like that?