Place the peeled, chunked 3 lb Yukon Gold potatoes peeled and cut into 2-inch chunks in a large pot and cover with cold water by at least 1 inch. Add 1 tbsp of kosher salt for the boiling water. Bring to a boil over high heat, then reduce to a steady simmer.

Cook the 3 lb Yukon Gold potatoes peeled and cut into 2-inch chunks until completely tender and a fork slides through with no resistance, about 18–22 minutes. Drain thoroughly in a colander and let them steam-dry for 2–3 minutes — this removes excess moisture and makes for fluffier mash.

While the 3 lb Yukon Gold potatoes peeled and cut into 2-inch chunks cook, melt 3 tbsp of unsalted butter in a large skillet over medium heat. Add the sliced whole leeks white and light green parts only and a pinch of kosher salt for the boiling water. Sauté, stirring occasionally, until softened and just beginning to turn golden, about 6–8 minutes.

Add the shredded 4 cups green cabbage cored and thinly shredded to the skillet with the 2 whole leeks white and light green parts only. Toss to combine and cook, stirring occasionally, until the 4 cups green cabbage cored and thinly shredded is wilted and tender but still has a little body, about 5–6 minutes. Season lightly with 1 tbsp kosher salt for the boiling water and ½ tsp white pepper. Remove from heat and set aside.

Warm the 1 cup whole milk and ½ cup heavy cream together in a small saucepan over low heat or in the microwave until steaming — do not boil.

Return the drained Yukon Gold potatoes peeled and cut into 2-inch chunks to the empty pot. Mash with a potato masher or pass through a ricer for an extra-smooth result. Add the remaining 3 tbsp of unsalted butter and mash until melted and incorporated.

Pour in the warm 1 cup whole milk and ½ cup heavy cream gradually, stirring and folding with a wooden spoon or spatula until the mash is creamy and smooth. You may not need all the liquid — add it until you reach your preferred consistency. Season well with 1 tbsp kosher salt for the boiling water and ½ tsp white pepper.

Fold the sautéed 2 whole leeks white and light green parts only and 4 cups green cabbage cored and thinly shredded into the mashed 3 lb Yukon Gold potatoes peeled and cut into 2-inch chunks until evenly distributed throughout. Taste and adjust seasoning — colcannon should be generously seasoned.

Transfer to a warm serving mixing bowl. Make a well in the center with the back of a spoon and add a generous knob of unsalted butter, letting it melt into a golden pool. Scatter sliced whole scallions thinly sliced over the top if using. Serve immediately.