Bring broth and butter to simmer in a deep large skillet.
Arrange whole bok choy evenly in skillet and simmer, covered, til tender, approximately 5 minutes. I like to flip the bok choy in the pan after 5 minutes, cover and cook for an additional 2 minutes.
Transfer bok choy to serving dish and keep covered. Meanwhile, bring the broth mixture to boil and cook until reduced to approximate 1/2 cup.
Stir in sesame oil and black pepper. Pour over the bok choy and serve.