Boil the yams with their peel on until tender. Let cool and remove peel.
Cut each yam into about four to five pieces and arrange in large baking dish. Pour orange juice over yams and the orange zest.
Sprinkle with brown sugar and dot with butter. Top with marshmallows and place orange slices on top, if desired.
Bake for approximately 20 to 25 minutes at 350F until marshmallows melt.