How to use corutines in a class

Hi, I'm trying to make a menu class with a fade function. My problem is that I can't used coroutines in my class, they just don't work and I don't get any errors in the console. I want to be able to write something like this:

myMenu.FadeOut();

or

yield myMenu.FadeOut();

I'm not sure, I've tried every combination I can think of but nothing.

this is my class:

 static class Menu {
 
     var menu //this is assigned in another part of the class
     
     function FadeOut(){
         while (menu.renderer.material.color.a > 0){
             menu.renderer.material.color.a -= 0.1;
             yield WaitForSeconds(1);
         }
     }
     
     //I have removed the rest of the functionality since it is irrelevant
 }

Is there a way I can do this? Any help would be great thanks!