Well, clearly your first mistake was letting her eat that hazelnut chocolate bar. I would've scarfed that thing away and eaten it myself.
Does this dcg speak English and understand well? Is her family's culture different enough to be causing a lot of confusion for her? Are there other kids in your dc, that she plays with? Do you think she gets enough sleep and good nutrition((for the most part)? What seems to be the catalyst for her tantrums; frustration, fights with the kiddos, etc.?
I think, if it were me, I'd get her some activities to do by herself throughout the day; could be groups and lots of noise around her could stress her out? Even if you simply set up a towel for her to sit on away from everybody else, give her magnetic dolls, lacing cards, dry erase board, anything to keep her busy and away from underfoot when you're trying to get things done. If you're working in the kitchen, are there things she can help you with?
Maybe designate a quiet corner for any child who just needs to take a break from it all. Soft cushions, fuzzy blankets, books, sensory jars, stress balls.
To me, if she's missing mom and trying to hang out with you a lot, it sounds like she needs some one on one attention and/or maybe just to escape from the group for awhile.