Preheat the oven to 350-degrees F.
Butter a casserole dish then set aside.
In a pot, add the water and the sweet potatoes.
Bring the sweet potatoes to a boil, then cover.
Note: you do NOT have to cover the sweet potatoes in water. As long as you keep the cover on, the water will steam the potatoes, and will be fork tender in no time!
Simmer the potatoes with the cover on for about 20 minutes, or until the potatoes are tender enough to mash.
Drain the potatoes and allow them to cool slightly.
Add the butter to a large bowl. (I like to use my mixer and the paddle attachment to mash the potatoes and mix all the ingredients together. However, you can mash them by hand with a potato masher if you like.)
Add the sweet potatoes to the bowl. The warmth from the potatoes will help melt the butter.
Add the brown sugar, vanilla extract, cinnamon, nutmeg, salt, and 1/4 of the chopped nuts. Reserve the rest of the nuts for later.
Mix all the ingredients together until the potatoes reach your desired consistency. To thin out the potato mixture, use a little splash of milk (optional).
Spoon the mixture into the prepared casserole dish. If you are making this ahead of time, STOP here and see the make-ahead instructions below.
Top the sweet potatoes with the remaining chopped nuts and mini marshmallows.
Bake for 15 minutes or until the mini marshmallows are toasted.