In S6 E15 of FRIENDS, Fat Monica slaps her Kit Kat bars before handing them to Chandler. Is this an enthusiastic gesture or is there a significance to doing this?
It's just an enthusiastic gesture.
She's slapping the Kit Kat bar lightly to draw Chandler's attention to it. In doing so, she emphasizes how "valueable" this is to her, and hints at the idea that her giving it to him is an important sacrifice on her part.
I dug up a clip of the action in question (here; for those who are interested, scroll down to the section titled "My last Kit Kat bar" - the slap happens right at the start of the short video clip.)
In the same way, a person might slap a magazine (back when we still read magazines) or a newspaper while discussing a particularly interesting/controversial article within with someone else. The purpose of the slap is really just to draw attention to the object being slapped, for emphasis.