Put the potatoes into a large pot, with the larges ones on the bottom, and add enough water to come halfway up the potatoes.
Cover the pot and bring to a boil over high heat.
2. When the water begins to boil, drain off about half of it, then return the pot to the heat, cover it again, reduce the heat to low, and let the potatoes steam for about 40 minutes.
Turn off heat, cover potatoes with a clean, damp tea towel and let sit for 5 minutes.
3. Melt 4 tablespoons of butter in a large skillet over medium-high heat.
Add kale or assorted greens and cook until just wilted, about 5 minutes.
4. Combine milk, scallions and remaining butter in a medium pot and bring to a simmer over medium heat.
Cook for about 2 minutes, then add the greens and stir well.
Remove pot from heat, cover and set aside.
5. Drain and carefully peel potatoes, then return them to the pot.
Add the greens and their liquid and mash until smooth, leaving a few small lumps in the potatoes.
Season to taste with salt and pepper.
6. To serve, push the back of a large soup spoon down in the middle of each portion to make a crater, then put a large pat of room-temperature butter into each one to make a “lake.”