Add the sweet potatoes to the bowl of the slow cooker.
Mix in the butter, heavy cream, eggs, brown sugar, and cinnamon. Stir to combine.
Place the lid on the slow cooker and cook on low for four hours.
Immediately before serving, top the sweet potatoes with the marshmallows. Place the lid on the slow cooker and cook for 3 to 5 minutes, until the marshmallows begin to melt. Serve immediately.
Notes
You may want to sprinkle chopped pecans on top, if you prefer nuts in your sweet potato casserole.