I suspect what has happened is that in keeping sweet things for once a week she has kept her sugar cravings and then seeing you get some chocolates it was just too much for her to resist.
Here’s something you could try, but it’s not for the faint-hearted! Try cutting out all sugar and “sugar-free” stuff (ie contains sweeteners) from her diet, including honey. No sugar-free choc spread, I’m afraid! No Nutella at Granny’s and no puddings at nursery/school. No chocolate or anything with sugar or sweeteners in the house. Research some actually sugar-free desserts and snacks that use fruit to sweeten (dates are great, as are ripe bananas) and make a selection for all of you to enjoy. Include as snacks, in lunchboxes, give to Granny. Expect her to turn her nose up at everything initially, but as her palate adjusts they will taste very sweet to her and you may find she starts to like them.
Next try including her in the cooking of the family meal. You can buy her some child-friendly utensils to use, get her her own apron, etc., make her feel special. I’d start with something like taco or pizza night, and she can choose what goes in/on them from a variety of options. Get her to mash up the guacamole and put what she wants in, and make it fun - play music, dance, goof around. This will start to change her associations with food as stressful (and as a way to control you, as by your reactions to her not eating she is being rewarded with attention) and get her interacting with it in a new way that gives her a bit more autonomy and creativity with food. Down the line you might think about taking her foraging or berry picking. See how she responds and take it super slow and gentle. If she still doesn’t eat anything then just be relaxed and chilled and try again another time.
Making food a sensual experience is really fundamental to good eating habits, so get her to smell things like herbs or freshly baked bread, make it a delight for the senses. Above all, avoid making her feel bad about not eating, just be cool and if it means she has no dinner, so be it. She’s hardly going to starve. Be totally zen, and try not to watch her as she eats. Tell funny stories, laugh, dance around, do silly impressions, just make dinner time a chance to bond and connect as a family and take the pressure off it being about what she’s eating.
With time, once she’s eating a bit more regularly you can reintroduce sugar if you choose, but I’d take it slowly. And definitely don’t punish her for eating your Mother’s Day chocolates, she’ll feel ashamed and that’ll only make it worse.