Place the cubed sweet potatoes in the crockpot.
In a separate bowl, whisk together the heavy cream, brown sugar, melted butter, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Pour the cream mixture over the sweet potatoes in the crockpot, stirring gently to coat.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the sweet potatoes are tender.
If using, sprinkle the mini marshmallows and chopped pecans on top during the last 30 minutes of cooking. Cover and let them melt and toast slightly.
Once done, mash the sweet potatoes with a fork or potato masher until creamy and smooth. Serve warm.