Bring broth and butter to a simmer in a deep large heavy skillet. Arrange bok choy evenly in skillet and simmer, covered, until tender, about 5 minutes. Transfer bok choy with tongs to a serving dish and keep warm, covered.
Boil broth mixture until reduced to about 1/4 cup, then stir in sesame oil and pepper to taste. Pour mixture over bok choy.