Though in the U.S. colcannon is often served on St. Patrick's Day with corned beef and cabbage, it's a traditional Irish potato dish eaten at Halloween, or Samhain (Gaelic Samhuinn). A common folk custom is to offer a bowl of colcannon to the fairies on Samhain, leaving it under trees such as hawthorns, which were particularly associated with fairies. Regional varieties of colcannon include using cabbage instead of kale or adding parsnips to the mash. You could use finely shredded Savoy cabbage instead of kale.
Peel the potatoes and cut into sixths. Place in a large pot of generously salted water and bring to a boil. Simmer until the potatoes are tender, then drain. Return the potatoes to the pot off the heat, cover with a clean tea towel and allow to sit for 5 minutes (this helps to dry out the potatoes, resulting in a fluffier mash).
While the potatoes cook, remove the tough center ribs of the kale and finely chop the leaves. Melt 2 Tbsp. of the butter in a large frying pan over medium heat. Add in the chopped kale and cook for about 10 minutes until softened. Remove from the heat and set aside.
Place the milk and remaining 4 Tbsp. butter in a small saucepan over a medium heat, or in a glass measuring cup in the microwave, until the butter has melted. Mash the potatoes with a potato masher, then pour in the hot milk mixture and beat with an electric mixer until fluffy and light.
Stir in the cooked kale and some salt and pepper. Transfer the colcannon to a serving bowl, make a small well in the top and add in a pat of butter.