Heat the butter and olive oil in a Dutch oven over medium-high heat. Sprinkle chicken thighs with salt and pepper. Sear chicken in batches until golden, 2 to 3 minutes per side. Remove the chicken to a plate and set aside.
Meanwhile, whisk together the chicken stock, soy sauce, sherry, brown sugar, hoisin and sesame oil in a small bowl and set aside.
Add the onions, chili paste, ginger, garlic and scallions to the pan and cook, stirring frequently, for 2 to 3 minutes. Pour in the sauce and deglaze the pan, then return the chicken to the pot. Add the butternut squash, bring to a simmer and cover. Cook until the chicken is cooked through, 20 to 22 minutes.
Remove the chicken to a platter. Add the kale, then stir in and allow to wilt. Spoon over and around the chicken, then garnish with cilantro leaves.