Set up Nomiku water bath and turn temperature to 85°C (185°F).
In a vacuum seal bag or zip bag, seal cubed sweet potatoes, butter, brown sugar and cinnamon sticks. Massage the bag and make sure the pieces of sweet potato lay flat. Seal according to vacuum instructions or using the water displacement method to remove air.
Sous vide for 1.5 hours. Remove from water bath and serve in personal heat safe ramekins or a baking pan.
Top with a little brown sugar and a generous amount of mini marshmallows. Right before service, use a culinary blowtorch or place dish under a broiler to brown and toast marshmallows.