Place the peeled, chunked potatoes into a large pot and cover with cold water. Bring to a boil over high heat and cook until the potatoes are tender when pierced with a fork, about 15 minutes.
While potatoes cook, heat a frying pan over medium heat. Add 1 tablespoon of butter, and sauté sliced leeks until translucent and fragrant, about 5 minutes. Add chopped greens and cook until wilted and tender, another 5 minutes. Remove from heat.
Drain the cooked potatoes and return them to the hot pot. Mash the potatoes with a masher or fork until smooth and fluffy.
Stir in the sautéed leeks and greens into the mashed potatoes. Pour warm milk and add 2 tablespoons of butter, stirring until the mixture is creamy and well combined. Season with salt and pepper to taste.
Transfer the colcannon to a serving dish. Garnish with extra melted butter if desired, and serve hot, showing a textured, creamy mash with bits of greens throughout.
