Start by preparing your potatoes. Place the peeled and quartered potatoes in a large pot and cover them with cold water by about an inch. Add a generous pinch of salt to the water. Bring to a boil over high heat, then reduce the heat to medium and simmer until the potatoes are fork-tender, about 15-20 minutes.

While the potatoes are boiling, prepare your greens. In a separate small saucepan, gently wilt the chopped kale or cabbage. You can do this by adding it to the pot with just a tablespoon of water and simmering for about 5 minutes until tender, or by sautéing it in 1 tablespoon of butter over medium heat for a few minutes until softened.

Once the potatoes are tender, drain them thoroughly in a colander. Return the drained potatoes to the warm, empty pot over low heat for about a minute to allow any excess moisture to evaporate. This step is crucial for fluffy mashed potatoes.

Mash the potatoes using a potato masher until they are mostly smooth but still have a little texture. Avoid over-mashing, which can make them gummy.

Gently warm the milk and the remaining 3 tablespoons of butter (and heavy cream, if using) in a small saucepan or in the microwave until the butter is melted and the mixture is warm. Do not boil.

Pour the warm milk and butter mixture into the mashed potatoes. Stir gently with a spatula until just combined and the potatoes are creamy. Season generously with salt and freshly ground black pepper to your taste.

Fold in the wilted kale or cabbage. Stir gently until evenly distributed throughout the mashed potatoes.

Serve the colcannon hot. For an authentic touch, make a well in the center of each serving and add a small pat of butter that will melt into the colcannon as you eat.