Peel potatoes. In a saucepan, cover with cold salted water. Bring to a boil and cook for 20 minutes or until done. Strain off water. Let potatoes dry, then hand mash. In a 2-quart pot, boil the kale in salted water until tender, about 25 minutes. In a large saucepan, over low heat, heat the milk with scallions, parsley, and thyme. Strain the chopped kale and add to milk. Simmer for 3 minutes. Add mashed potatoes to kale, stir in 4 tablespoons butter, and mix to a creamy consistency. Correct seasoning. Place into a serving dish, making a well in the center. Fill generously with remaining butter and serve.