In a saucepan or stock pot, cover potatoes with water and bring to a boil, cooking until fork tender, about 30 minutes if left whole. If desired, cut and peel before this step to lessen cooking time.
Meanwhile, cook bacon over medium high heat in a skillet that has a lid. Remove the cooked bacon from the skillet and set aside to cool. Sauté the cabbage or kale in the bacon drippings until soft and translucent, covering with the lid to somewhat steam the vegetables.
Drain the cooked potatoes. Peel, if that hasn't already been done, then mash with 1/4 cup milk and 4 tbs butter. Crumble the bacon and add it to the potatoes. Add the cooked cabbage as well. Stir until smooth and evenly mixed, adding more milk if needed to help the texture.
Serve in bowls, making a well in the center to put more butter, if desired.