Yes, I know Marsala is in, but butter in coffee? hmmmm... did I miss this one? I prefer to add a spoon full of coconut oil (OR homemade Kaklua), but I guess if I'm out, then butter is a good alternative? Read up a little on bulletproof. Eh, I think I'd puke a little if I drank that.
Coconut oil only for me. Yes, I still use it for many things. And no new updates on all of my experiments with it. Only in the fact that I added coffee creamer to my list awhile ago. It's healthy, delicious and the smell takes me to the beach.
...or if you prefer a butter latte, then go for it.