How do you add ingredients to fudge without stiring it? There have been a couple of questions recently about making fudge, and a comment which came up twice in answers has got me wondering about something.  (I'll admit, both answers are by Sobachatina, but I think my question could be answered by anyone.)  Both the recipe for fudge I grew up with, and another recipe I found online instruct ingredients to be added as the fudge is taken off the heat (butter and vanilla, in the linked recipe).  But Sobachatina seems very adamant about this being a BAD time to disturb the fudge, as it will cause crystal formation.  So, if I'm not supposed to stir the fudge, how do I add the butter and vanilla?
Butter is typically added when the fudge is first taken off the heat- but it isn't mixed in. The butter is allowed to melt across the surface to keep it from forming a skin on top.

Vanilla, nuts, and all other additions are mixed in at the end of the cooling period when the fudge is stirred.

Alton Brown, with some help from Shirley Corriher, explained the process well.