In a large pot, simmer kale, covered, in 2 cups water and the oil for 10 minutes.
Drain and chop fine; set aside and keep warm. In a small pot, bring the potatoes and water to cover to a boil and simmer until tender.
In another small pot, simmer the leeks, covered, in the milk for 10 minutes, and keep warm.
Drain the potatoes and puree them, using a potato ricer or masher, into the large pot.
Add the leeks with their milk, and the cooked kale.
Beat with a wooden spoon until fluffy.
Season with mace, salt and pepper.
Mound on a plate and top with melted butter.
Garnish with parsley.