Boil yams for 20 minutes in a large pot, or until they are easily pierced with a fork, and drain the water from the pot.
Spray a 9x13 inch baking dish with nonstick cooking spray and spread boiled yams inside.
Distribute small pieces of butter and cinnamon evenly over the yams.
Sprinkle brown sugar then miniature marshmallows over yams.
Bake in the preheated oven for 25 minutes, or until sweet potatoes are tender and marshmallows have melted.
Notes
Yams and sweet potatoes are not the same, though some people tend to use them interchangeably. Yams have a harder brownish outside and a little more starchy inside. Yams can also grow to be very large. Where as the sweet potato has a lighter reddish brown outside and more of a softer buttery inside. Sweet potatoes tend to stay around the same size, but maybe slightly larger than a russet potato.