Combine the sweet potatoes, 4 tablespoons butter, 2 tablespoons cream, and 1/2 teaspoon salt in a 3-quart saucepan over high heat until the butter and cream start to bubble. Reduce heat to med-low (on my stove, it's a 2 of 10, just above the lowest setting), cover and cook for about 30 minutes, or until potatoes are soft and tender.
While the potatoes are cooking, wilt the kale in a skillet with a light coating of oil over medium heat about 2 minutes. Set aside until ready to stir into potatoes.
Use a potato masher to mash the cooked sweet potatoes right in the pot, then stir in the 2 cups chopped kale and all but 1 tablespoon of the chopped chives. The heat from the potatoes should wilt the kale. If not, cover and allow to sit for 5 minutes.
Add salt and additional cream, if needed, and transfer to a serving dish. Garnish with chives and pat of butter on top.