Eating only half a pepernoot and putting it back in the bowl is crazy behavior.

In this case the gesture is to not speak with a mouth full of food, which is understandable.

Also, she made the gesture twice without saying "mmmmm" while making it. Imo, that sound is part of the gesture.

Italians have a gesture for this as well, and it speaks for itself: nobody says "mmmm" while doing it.