Place the potatoes in a large pot, cover them with water, and generously season with salt. Bring the water to a boil and cook the potatoes until they become completely soft, which usually takes around 10 to 15 minutes. Drain the water and return the potatoes to the pot.
In the meantime, melt 4 tablespoons of butter in a large pot. Add the scallions and garlic, and cook them until they begin to soften and release a fragrant aroma, which should take approximately 2 minutes. Next, add the shredded kale, milk, and cream to the pot. Allow the mixture to cook for about 4 minutes, stirring frequently, until the milk reaches a simmer and the kale wilts.
Using a potato masher, mash the kale mixture into the potatoes until you achieve a smooth consistency. Season with salt and pepper according to your taste.
To serve, top the dish with the remaining 2 tablespoons of butter and sprinkle some additional cracked pepper on top.