01 - Place the potatoes in a large pot and cover completely with cold water. Add 1 teaspoon of salt and bring to a rolling boil. Reduce the heat and simmer until the potatoes are fork-tender, typically 15–20 minutes.
02 - While the potatoes cook, melt 3½ tablespoons of the unsalted butter in a large skillet over medium heat. Add the chopped kale and sauté for 3–4 minutes, or until it has wilted and become tender. Stir in the thinly sliced scallions and cook for an additional 1–2 minutes. Remove from heat and set aside.
03 - Thoroughly drain the cooked potatoes and return them to the same pot. Mash vigorously until they are smooth and fluffy, ensuring no lumps remain.
04 - Gently warm the whole milk until it is hot but not boiling. Add the warm milk and the remaining butter to the mashed potatoes, mixing until completely creamy and well incorporated.
05 - Carefully fold the sautéed kale and scallion mixture into the creamy mashed potatoes. Season generously with additional salt and freshly ground black pepper to taste, adjusting as needed.
06 - Serve the colcannon immediately while hot, optionally topped with an extra pat of butter for added richness.