Thanks for the advice! I ended up thawing another stick of butter to avoid any issues with the icing. Made a Swiss buttercream that turned out great! :)

In my experience buttercream is a bit fussy - I'd avoid the melted butter. It's a real bummer when your frosting "breaks" and you have a grainy, soupy, mess!

That depends on how much of it is melted. If possible, remove the more solid parts for the frosting, drain off the melted parts and use the melted butter for cooking something else.

Softened, of course, but not melted because the emulsion has been broken.

imho, softened butter works better than brick hard.

The butter can be soft (room temp) but not melted as in liquid.