Chocolate is actually poisonous because of one of the chemicals in it: "too much" is not very much at all, depending on the type of chocolate it could be smaller than a regular chocolate chip. Its not so much the sugar, although sugar is bad too!
Zissou also is a begger and a scrap eater. She eats plenty of her own food, and when she gets mine it is smaller than the fingertip of my pinky finger. So, she gets a little taste. I wouldn't give her a whole chip, though. I asked my vet about it last time she was in and he said as long as it isn't compromising her diet (spoiling her supper, you could say) it was fine to give her a little bit of bread crust, cheese, broccoli (yeah, weird), etc every now and then.
It is a very bad thing when a cat eats only table scraps and no cat food, but I don't think its terrible for them to taste what we're eating sometimes.
As for the behavior you describe, I wouldn't tolerate that, and you can train her not to do that. What if you are eating, say, chocolate chip cookies, and leave the room? Also, if you have someone over for dinner, a lot of people would be offended by a cat who stuck her face in their plate! I never give Zissou anything if she's actively begging or trying to get at it, and only if she just comes up, sits on the arm of the chair and looks at me, or on the floor, or when she's not begging at all. If she really tries hard (she has actually knocked a bowl of cereal out of my hands before) such as touching the plate or trying to grab out of my hand, I blow on her face. Just one quick puff of air, and she retreats back about four inches. She's getting better!