I see alot of people are posting about the right or wrong of your choice, and about letting it go, And that may be very good for this instance. However, In your post is a deeper question, it is all well and good to forgive a chocolate , the issue is can you trust your mother to follow your wishes.
I would suggest that you talk to your mother in these terms, She was a mother to you, How would she have felt if your babysitter had let you stay up past bedtime, or get ou tof doing your homework.
Be patient, explaine to her that it is not so much the sweet which upset you but the disregard for your wishes. Be willing to compromise, allow that if she talks with you, you can make Grandma's house a spcial place where some of the rules dont apply.
(My kids always got to eat fast food kids meals at Granny's, Never at Home)
If you feel like you are still a little intimidated by your mother (and who isnt) you can always find allies. A letter from your pediatrician about the importance of healthy eating habits might be great. You might also consider the other grandmother as an ally. If you have a good relationship She may be able to talk with your mother grandma to grandma.
I belive that as the mother you need to stand up for yourself and your views. You and the father have the final say in how this child is raised, and although you will listen to your mothers advice, her wealth of wisdom and experiance, do not make her the childs mother, nor can her love of her grandchild, however great that may be, ever come close to your love of your daughter,
remember, there are as many right-ways to parent as there are good parents in the world, we as human beings have been doing this a long long time, and most of us seem tocome out okay, in spite of our parents love.