Yes. People are putting butter in their coffee.
The practice started in Tibet with buttered tea. They brew the tea, then float yak butter and Himalayan salt on top. Dave Asprey brought the concept to the U.S. but used coffee instead of tea. He uses grass-fed unsalted butter. He has developed Bulletproof Coffee.
If you want to try butter coffee at home without purchasing Bulletproof coffee, put 1 cup brewed coffee in a blender with 1 tsp organic grass-fed unsalted butter and 1 Tbsp coconut oil and mix until it froths. Enjoy.