Bring broth and butter to a simmer in a deep heavy skillet. Arrange bok choi evenly in skillet and simmer, covered, until tender, about 5 minutes. Transfer bok choi with tongs to a serving dish and keep warm, covered. Make sure all the liquid from the serving dish is drained off into the skillet. Boil broth mixture until reduced to about 1/4 cup, then stir in the sesame oil and pepper to taste. Pour mixture over bok choi.
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