Preheat the oven to 350 degrees F. Roast the potatoes on a sheet pan until fork tender, and then let cool. When the potatoes are cool enough to handle, halve lengthwise with a serrated knife. Carefully scoop out the guts of the potatoes, using a melon baller, and then run the guts through a ricer into a glass mixing bowl.
Preheat the broiler.
Melt the butter in a large saute pan over medium-high heat. Add the scallion whites, sprinkle with salt and sweat until tender. Stir in the kale, sprinkle with more salt and cook until wilted. Remove from the heat and let cool slightly. Combine the kale with the mashed potatoes, and mix in the sour cream until combined. Fill the potato skins with the mashed potatoes and kale.
Place the potatoes on a sheet pan under the broiler just until the tops are crunchy. Garnish with the scallion greens.