In a mixing bowl, whisk together the flour, baking powder, baking soda, salt, garlic powder, and sugar. Add cold sour cream, milk, and melted butter. Gently fold the mixture until a cohesive but soft dough forms; do not overwork.
Return the chopped chicken and any collected juices to the pot. Reduce the heat to low and stir to incorporate evenly.
Using a small scoop or spoon, form dumplings and place them gently over the surface of the simmering broth in an even layer. Spoon a bit of the hot liquid over each to moisten.
Securely cover the pot with a lid. Slightly raise the heat to maintain a soft simmer. Cook undisturbed for 15 minutes to allow the dumplings to steam through.
After 15 minutes, remove the lid and test the center of one dumpling with a toothpick. If it emerges clean, the dumplings are cooked. If not, re-cover and simmer a few minutes longer.
Garnish with freshly chopped parsley, if desired, and serve hot.