You don't actually need an electric mixer (I just used a spoon), and I used salted butter and omitted the pinch of salt, but it turned out really, really well. It's very much a "sit and enjoy in front of a fire while it snows outside" sort of drink, like mulled cider or wine. Pair it with some other fall/christmas dessert like bread pudding or pumpkin pie, and you've got yourself a good time.
Dude, this looks amazing. I really wish I would have known about this around Christmas time. I'm going to have to buy the ingredients and give this a shot.