In a 6-quart Dutch oven over medium heat, melt 3 tablespoon butter. Add the celery, carrots, and onion. Cook for 5-7 minutes, stirring occasionally, until the vegetables begin to soften.
Place the raw chicken breasts on top of the vegetables. Add the bay leaf, poultry seasoning, garlic powder, salt and black pepper then pour in chicken stock.
Bring the liquid to a gentle simmer over medium heat. Once simmering, reduce the heat to low. Cover the Dutch oven and gently simmer for approximately 20 minutes, or just until the thickest part of the chicken breasts reaches 165°F. *Avoid a vigorous boil. A gentle simmer helps keep the chicken tender and prevents the broth from reducing too quickly. Transfer the cooked chicken to a plate or cutting board and remove the bay leaf. Leave the vegetables and broth in the Dutch oven. Shred chicken with two forks or cut it into bite-sized pieces.
Keep the Dutch oven covered on medium heat to maintain the broth at a gentle simmer while preparing the thickener and dumplings.
In a medium bowl or measuring cup, whisk together the cold milk and cornstarch until completely smooth.
Slowly pour the cornstarch mixture into the simmering broth while stirring constantly. Continue stirring for 2-3 minutes or just until the broth begins to thicken enough to coat the back of a spoon. *Do not allow it to reach a hard boil, think gently bubbling. Return the shredded chicken to the Dutch oven. Gently stir the chicken and vegetables through the creamy broth. Taste and adjust seasonings, if needed.
While the soup is thickening, prepare the dumpling dough. In a medium bowl, whisk together the flour, baking powder, and seasoned salt.
Slowly add the buttermilk and melted butter, stirring just until a soft dough forms. If the dough is too dry or crumbly, add 1 tablespoon of buttermilk at a time, if needed.* Do not overmix the dough. Reduce the Dutch oven heat to medium-low so the soup maintains a gentle simmer. Lightly coat a tablespoon, large measuring spoon or a small cookie scoop with cooking spray. Scoop small portions of dough and gently drop them into the hot liquid.
Gently mix the dough balls into the creamy broth. Place the lid on the Dutch oven and cook for 15-20 minutes.*Do not repeatedly lift the lid. The trapped steam is essential for cooking the dumplings properly. The dumplings should be puffed, firm, and cooked through in the center. If you are unsure if the dumplings are cooked through, remove one dumpling and cut it open. The center should no longer be wet or doughy. Turn off the heat once the dumplings are fully cooked. Remove pot from the heat. Let the chicken and dumplings rest, covered, for 5 minutes before serving.
Ladle into bowls and serve hot and top with fresh parsley, chives or thyme.