Preheat the oven to 375ºF. Line a rimmed baking sheet with parchment paper. Toss the sweet potato cubes in the olive oil and ½ teaspoon of the salt. Spread them out on the prepared baking sheet. Roast for 45 minutes, or until cooked all the way through.
While the potatoes are in the oven, season and sear the chicken: Melt 1 tablespoon of the butter in a large sauté pan that has a matching lid over high heat. Season the chicken with ½ teaspoon of the salt and the black pepper. Once the butter is melted, sear the chicken on one side for 4 minutes, or until it develops a slight brown color. Flip the chicken over, sear for an additional 3 minutes. If using a slow cooker to finish cooking the chicken, jump ahead to Step 4.
To cook the chicken on the stovetop: In the same pan, cover the chicken with the water, cover, and simmer over medium heat for 20 to 30 minutes.
To cook the chicken in a slow cooker: Transfer the seared chicken to a slow cooker, cover with water or broth, cover, and cook on high for 3 to 4 hours or on low for 6 to 8 hours.
Pull the chicken from its cooking liquid and place in a large bowl. Using two forks or an electric mixer, shred the chicken into small pieces. Pour in ½ cup of the Buffalo sauce and toss to combine. Set aside.
In a separate sauté pan, melt the remaining tablespoon of butter. Add the kale and sauté for about 8 minutes, or until the color is deepened and each piece is wilted. Season with the remaining ½ teaspoon salt and 1 tablespoon of the lemon juice.
Preheat the oven to 350ºF. In a 9 by 13-inch baking dish, or similar size, spread the roasted sweet potatoes out evenly along the bottom. Top with the kale and then with the buffalo chicken last. Drizzle the remaining ¼ cup of Buffalo sauce on top of the chicken and sprinkle the bacon over the top. Place in the oven and bake for 25 minutes, or until heated through.
Whisk the ranch dressing with the remaining tablespoon of lemon juice and drizzle it over top. Garnish with the green onions and serve warm.