Place the chicken thighs in the slow cooker, spreading them out evenly so they cook uniformly.
In a small bowl, whisk together the cream of mushroom soup, chicken broth, onion powder, garlic powder, and black pepper until smooth and well combined.
Pour the gravy mixture over the chicken, making sure all the pieces are coated thoroughly. Use a spatula to gently lift and distribute the sauce if needed.
Cover the slow cooker and set it to low. Let it cook for 6 to 8 hours until the chicken is tender and falling apart, filling your kitchen with a savory aroma.
Check the chicken after about 6 hours. It should be juicy and easily shredded with a fork. If the gravy seems too thin, remove the chicken and set aside.
Turn the slow cooker to high. In a small bowl, mix 1 tablespoon of cornstarch with 2 tablespoons of cold water to make a slurry, then stir it into the hot gravy. Let it simmer for about 10 minutes until the gravy thickens and becomes velvety.
Return the shredded chicken to the thickened gravy and stir to coat well. Let it sit for another 10 minutes to absorb flavors and heat through.
Finish by stirring in the butter until melted, giving the gravy a glossy, rich finish. Taste and adjust seasoning if needed.
Serve the hot chicken and gravy over mashed potatoes, rice, or crusty bread for a cozy, satisfying meal. Garnish with fresh herbs if desired.