In a medium bowl, whisk together brown sugar, butter, spices, salt and vanilla.
Pour over sweet potatoes and stir to combine.
Cover and cook on low for 7-8 hours or on high for 3-4 hours or until sweet potatoes are tender.
Remove lid and mash sweet potatoes.
Top with marshmallows.
Cover with lid and continue cooking for 5-10 minutes to melt marshmallows.
Sprinkle with pecans and serve immediately.
Notes
To mash potatoes, you can also remove them from the crockpot and mix them with a stand mixer. If you would like browned marshmallows, place insert of the crock pot into the oven at 375 degrees for 5-10 minutes, watching carefully.