Boil the potatoes in salted water until tender, mash them and set aside. For extra fluffy potatoes run them through a ricer.
Meanwhile, cut the greens and root end off the leeks and discard. Cut the white parts in half vertically. Fan out the leaves and thoroughly rinse them under running water. Slice the whites crosswise.
In a large skillet, sweat the leeks and cabbage with some salt and pepper in half of the butter until softened. Watch the heat and cook them gently. Avoid burning the butter or browning the vegetables.
With a spatula, scrape out the leek/cabbage combination and all of the butter from the skillet into the pot of mashed potatoes.
Add the remaining butter, cream and additional salt and pepper if necessary.
Mix everything with a spoon and return it to the stove to heat everything through.
Serve with an ample amount of Yuletide joy with family, friends and the Ghost of Christmas Present.