I've never tried it but I will next time I have coffee
Discussion
If you can get a mini hand blender or throw it in a small ninja-like blender the better. It will make it nice and creamy, otherwise the melted butter will coalesce at the top with an oily sheen that may be off putting.
I cut thin slices off and drop them into a mug, then stir vigorously with a fork as I pour the hot coffee over it.