Add cubed sweet potatoes to a large pot and cover with water. Make sure there is enough room for water level to be higher than potatoes by at least an inch. Over high heat, bring to a rolling boil. When potatoes are easily pierced by fork, drain potatoes and return to pot.
Add brown sugar, butter, cream, salt, vanilla, cinnamon and pumpkin pie spice to potatoes. Mash either by hand, or using an electric mixer for a smoother texture.
Spoon mixture into slow cooker set on low temperature.
In a bowl, combine flour, brown sugar, rolled oats and cinnamon. Stir to combine. Pour in melted butter and stir to combine until a crumb like texture develops. Sprinkle over potato mixture in slow cooker. If desired hold back 1/4 cup of mixture to add to top of marshmallows. Sprinkle marshmallows over topping. Place lid on slow cooker and cook for at least 30 minutes. After 30 minutes, switch slow cooker to warm setting to keep warm until serving.