Method:
Place potatoes in a large saucepan and cover with cold salted water by 2 inches. Bring to a boil over high heat, then lower the heat and simmer until they are very tender, 15 to 20 minutes. Drain thoroughly. Return potatoes to the pot and add butter and milk. With a potato masher or large fork, mash potatoes until very smooth.
Meanwhile, if using frozen kale, defrost according to package directions. Squeeze kale gently with your hands to remove as much water as possible. Add kale, green onions, salt and pepper to potatoes and stir until combined.
Heat a large skillet over medium heat and add oil. When hot, scrape the potato mixture into the pan and use a spatula to level the top. Cook until golden brown on the bottom, 8 to 10 minutes. Working in sections (the cake will break apart as you flip it) use a spatula to flip the pancake over, then press the top gently with the spatula to make the top smooth again. Continue cooking until the bottom is browned and the potatoes are heated through, about 8 more minutes. Serve warm.