If using raw sweet potatoes, peel and cube them. Cook them in boiling water until tender. Then mash.
If using canned yams, drain and mash them.
Add the sweet potatoes to the bowl of the slow cooker.
Then 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 4 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.