Place the potatoes in the slow cooker and add the stock. Cover and cook on high for about 2 hours, until the potatoes are tender.
Transfer the potatoes to a bowl and mash with a potato masher. While the potatoes are still hot, add the milk, scallions, and 4 tablespoons of the butter. Mix well and season to taste with the salt and pepper.
Place the remaining 2 tablespoons butter on top of the potatoes to melt and serve immediately.