Place the diced sweet potatoes in a large bowl, and set aside.
In a small saucepan, whisk together the butter, brown sugar, maple syrup, cinnamon, and nutmeg. Place over medium-low heat and continue to whisk until the butter and sugar are dissolved and smooth.
Take off the heat and stir in the vanilla.
Pour this mixture over the potatoes and stir to combine.
Spray the body of a 6-quart slow cooker with cooking spray. Pour in the potato mixture.
Cook on low for 6-8 hours or high for 4-5 hours, stirring every 30 minutes or so to prevent burning. Cook until potatoes are softened, and the liquid is thickened.
Add half of the marshmallows and stir them in.
Add the remaining marshmallows on top.
You can let them melt by placing the cover back on for a few minutes, or you can broil them until browned and serve immediately.
Notes
The marshmallows are a great addition but can be left out.
This can be made smoother, see above on how to do that.
You do not need to precook the potatoes before adding to the crock pot.