Butter usage in fudge
I'm not a professional fudger, but here is my theory: one should add it to the boiling mixture.  The reason is that butter has milk solids that are said to  “burn” at low temperatures (somewhere in the range 120C-150C or 250F-300F) which just above the soft ball stage (113C or 235F) needed for making fudge.  I interpret the burning to mean that those milk solids (sugars and proteins) get a chance to contribute to the Maillard reaction that gives caramel (and its grained cousin the fudge) its great and rich flavor.   Given how unique and varied the flavors arising from butter are, I would not want to miss their contribution by adding them after the heat is gone.