How to rotate bone perpendicular to plane with parent bone If I have two bones, with one parented and connected to the other, How would I rotate the second bone around the axis perpendicular to the plane with it and the parent?

For instance, in the following image the desired axis of rotation would be the global Y-axis.



Obviously in this situation I could just hit R > Y, but how would I do this in a situation where the bones are not nicely lined up with the world axes?

Edit:

Here is something I have noticed, when I rotate a bone or selection of bones, it seems to only rotate the heads/tales of the bones around the pivot point, it doesn't actually rotate the bones themselves.  Here is an image describing what I mean.  I took 2 bones, duplicated them, and rotated each one a different amount.  Notice how in each case, the second bone has not rotated, it's sides are still parallel to the x and y axes.  It seems that if the bone actually rotated then rotating on the local coordinates of the bone would do what I want.
A bone and it's parent form a triangle, you can snap a polygon to it with vertex snap:



Next switch to Normal gizmo/manipulator and save the transform to use it on the bones:



It is not very convenient, maybe there is addon for it but I rarely need to do this.