Stir the brown sugar, butter, water, cinnamon and vanilla together in a crock pot then add the sweet potatoes and stir to combine once again.
1½ cups brown sugar, 6 tablespoons unsalted butter, 2 tablespoons water, 1 teaspoon ground cinnamon, 4 pounds sweet potatoes, 1 teaspoon pure vanilla extract
Cover with the lid and cook on high for 3 ½-4 hours or on low for 6-8 hours or until the sweet potatoes are tender, stir every hour to prevent burning.
Stir in 2 cups of the marshmallows and ½ cup of the pecans then sprinkle the top with the remaining pecans and marshmallows.
3 cups mini marshmallows, 1 cup chopped pecans
Cover once again and let the marshmallows melt on high for another 10-15 minutes.
Uncover and let the sweet potatoes cool for about 20 minutes before serving.