How do I hook up editor extensions to Unity Editor start-up?

I want to create a simple editor extension.
I consists of only of a few buttons in the Window tab via the [MenuAttribute(string itemName)]. The buttons should be “check-able”, representing boolean data. It works - that is not the problem.
But how do I save that data on a per project basis (I could figure out a way, but maybe there is some pre built functionality for this) and more importantly: How do I set those values visually based on the saved data on editor start-up?

Bump - haven’t found a solution yet