Spray the slow cooker insert with non stick cooking spray. Chop the potatoes into 1 inch pieces and add them to the crock pot. Sprinkle the salt evenly over the potatoes and add a good amount of pepper. Pour 1½ cups milk over everything. Cook on high for 4-5 hours or until potatoes mash easily with a fork. If you prefer, remove any browned crusty bits from the side of the slow cooker. Mash the potatoes with a potato masher or use a hand mixer to whip until you get the smoothness you desire.

Warm the butter and additional 1 cup milk until the butter is melted and liquid is warm to the touch but not scalding. Slowly stir in the butter and milk, rotating with the sour cream. Use a potato masher to work the liquid into the potatoes. It will be soupy at first, but the potatoes will soak up the liquid as you mash them in. Add the last ½ cup of milk and sour cream if you want them to be creamier. Taste for seasonings and add more salt and pepper if needed. Stir in the chives. To serve, melt more butter over top and sprinkle with chives.