01 - Place the peeled and cut potatoes in a large pot and cover completely with cold water. Add 1 teaspoon of salt and bring the water to a rolling boil. Reduce the heat to a gentle simmer and cook until the potatoes are fork-tender, approximately 15 to 20 minutes.
02 - While the potatoes are cooking, melt 3 1/2 tablespoons of the unsalted butter in a large skillet over medium heat. Add the chopped kale and sauté for 3 to 4 minutes, or until the leaves have wilted and are tender. Stir in the thinly sliced scallions and continue to cook for an additional 1 to 2 minutes. Remove from heat and set aside.
03 - Once cooked, thoroughly drain the potatoes and return them to the empty pot. Mash vigorously with a potato masher until they are completely smooth and fluffy, ensuring no lumps remain.
04 - In a separate small saucepan, gently warm the milk until it is hot but not boiling. Add the warm milk and the remaining 3 1/2 tablespoons of butter to the mashed potatoes. Mix thoroughly until the potatoes are creamy and the dairy is fully incorporated.
05 - Gently fold the sautéed kale and scallion mixture into the creamy mashed potatoes. Taste and adjust the seasoning with additional salt and freshly ground black pepper as needed.
06 - Serve the Colcannon hot, garnished with an optional extra pat of butter on top for added richness.