In a pot, put all the potatoes, cover with enough water, and add salt (I usually put coarse salt in the palm of my hand, about ¾ teaspoon of salt). Bring to a boil. Cook the potatoes until tender (around 20 mn).
Meanwhile, take a big saucepan and bring some water to boil. Add cabbage and leek into it. Reduce the heat but keep a low, rolling boil until the cabbage, and leek are a little softer (about 3-5 min.). Add the spinach and cook one more min. Drain well, then put it back in the saucepan. Add 2 tablespoon of butter, cover, put aside.
When the potatoes are cooked, drain the water, and return the potatoes to the saucepan. Set the burner to low, and remove the lid so moisture can evaporate. When the potatoes are perfectly dry, add the milk to the saucepan, with 2 tablespoon of butter. Wait until the milk is warm (but not boiling), the butter has fully melted, and the pot is starting to steam.
Mash the potatoes with a potato masher or a fork. Then mix the greens (kale, leek, spinach) into the mashed potato. Season with salt, pepper, and sprinkle with chives or parsley. Add the last 2 tablespoon of butter, let melt, and mix gently before serving.