Several years ago, a friend from Ethiopia suggested I try butter in my coffee. He said it reduces the bitterness.
It did and is now my favorite way to drink it.
My question is, is this really Ethiopian or do other cultures use butter in coffee?
Not specifically butter, but to the extent that butter is cream and salt: A pinch of salt will make coffee less bitter — and I’ve heard that trick referred to as an “old church social” and also a “military” thing so I’d say part of your (salted) butter trick is widespread. And cream is also widely used.
Both-in-one does sound fairly unique though.
Recently the health community CrossFit etc have discovered the benefits of buttered coffee. They generally use coconut oil, but it's a similar thing. Just google "coconut oil in coffee paleo" & you'll find tons of info, some good, some ??? ;-)
I'm from Poland, and have heard of someone's grandma from a rural area drinking coffee this way. So it may not be just Ethiopian way of serving that beverage.