Cook the sweet potatoes in boiling water until you can stick a fork in them - times will vary depending on sizes of potatoes. Set aside until they are cool enough to handle.
Preheat oven to 350° Fahrenheit unless you are baking them with a roast, they can be baked at any temperature to bake with a roast.
Once the sweet potatoes are cool enough to handle, peel them and cut into ¼ inch slices.
In a small saucepan heat melt butter until it is most of the way melted. Add in brown sugar and cinnamon. Stirring often, heat over medium heat for about 1 minute or until the mixture is mixed well.
Place about half of the sweet potato slices on the bottom of a casserole dish - either 2½ or 3 quart is best. It's ok if it is slightly more then a single layer.Drizzle about half of the butter mixture over the slices. Repeat layers once.
Spread the miniature marshmallows over the top of the casserole. Use at least enough to cover evenly but you can add more if you like.
Place in oven at 350° Fahrenheit and bake for about 25-30 minutes, until the potatoes are heated through and marshmallows have the desired amount of browning.If baking at 325° Fahrenheit, add an extra 5-10 minutes.
Notes
Nutritional values are estimated and so are serving sizes. If your sweet potatoes are different sizes, boil the larger ones for a couple minutes before adding the smaller ones. You may need to carefully remove the smaller ones sooner as well.For the marshmallows - miniature are best but if using regular or large, you can cut them into pieces with kitchen scissors. 2 cups is estimated and the amount varies depending on the shape of your casserole dish. Use enough to cover the casserole and more if desired.If using unsalted butter, I recommend adding about a quarter teaspoon of salt to enhance the flavor.This can be made the day before. If using a glass dish, remove from fridge about 20 minutes before baking to prevent a drastic temperature change on glass. Add about 5-10 minutes to cook time.