I think you're getting a somewhat hard time on here OP.
As a manager you're required to optimise the performance of the team as a whole.
However, what on face value might seem a straight forward issue to resolve can often create a significant ripple effect.
On an individual level it would seem that her overall performance outweighs the negative impact on her breakfast ritual.
However, the wider consequence of that is her impacting the morale and efficiency of other team members by refusing to engage in her work responsibilities until her "ritual" is complete.
Whilst it's right to consider the knock on consequences of approaching her wrt opening a can of worms re: x person calling home ever hour or y person spending an inordinate time in the bathroom, I don't think that can be justification for not addressing the issue she is creating.
Rather, these are totally separate concerns that may/may not be legitimate that you need to address in addition to this specific employees behaviour - as opposed to letting sleeping dogs lie. If she raises such issues, then you simply say you'll look into them but it's not relevant to the discussion you are having about her behaviour.
The fact she doesn't respond well to criticism is an issue in itself. Being good at your job isn't solely about doing it well in isolation.
You're not a good employee if you are negatively impacting the productivity and morale of the rest of the team and this is the element I think your missing when considering your course of action. This isn't a case of giving a good employee some leeway, rather it's addressing the behaviour of an employee whose behaviour is poor.
You need to speak with her privately and make clear that she needs to come in 30 mins earlier if she wants to continue her breakfast ritual or otherwise knock it on the head.
Please do not send an office wide email on the subject. Everyone will know who it's aimed at and that's just akin to a public shaming.