Mix together the butter, brown sugar, cinnamon and vanilla extract together in a small bowl.
Place the diced sweet potatoes in a 4-6 quart crock pot. Stir in the butter mixture and toss the sweet potatoes to coat the sweet potatoes in the butter mixture. Top it with the chopped pecans.
Cover and cook on low for 7-8 hours or high for 3-4 hours until the potatoes are soft.
Place the marshmallows on top for the last 10-15 minutes to softly melt the marshmallows.