Heat a large pot over medium heat. Add butter to melt and stir in carrots, celery and onion. Frequently stir the vegetables until they become slightly tender about 6 minutes.
Add the two tablespoons of flour to the vegetables. Continue to stir to cook the flour for 1-2 minutes.
Add the broth, bay leaf and thyme to the vegetables and bring to a boil. The broth will begin to thicken from the flour that was added.
Meanwhile mix the flour, baking powder, baking soda and salt together in a bowl.
Add the melted butter, milk and sour cream into the flour. Stir until just combined and no white streaks of flour remain.
Reduce the turkey and vegetables to a simmer and begin to drop the dough by the spoonful over the top of the broth.
Cover the pot and simmer for 15 minutes. Do not uncover during the 15 minutes because the steam the lid creates cooks the dumplings as they float on the top of the broth.
After the 15 minutes, uncover and test with a toothpick to see if they are cooked through and not doughy. If needed return lid and cook for an additional couple of minutes.
Garnish with fresh parsley and serve.