Boil potatoes in a large pot of salted water until tender, about 10 minutes. Drain and transfer potatoes to a large bowl. Add 2 tablespoons butter and lightly mash the potatoes.
Boil kale and leeks in a large pot of water until tender, 5 to 7 minutes. Drain and transfer kale and leeks to a blender. Add white parts of the green onions and 2 more tablespoons butter; blend until smooth, scraping down sides as needed, 1 to 3 minutes.
Stir pureed kale mixture into the bowl of potatoes and continue to mash. Season with salt and black pepper to taste. Add cream and stir until desired texture. Top with 2 tablespoons butter and green parts of the green onions.
You can substitute kale with other leafy greens such as Swiss chard or cabbage.
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