In a stock pot over medium heat, melt butter. Add onion and celery and cook until lightly browned around the edges, about 4 to 5 minutes. Add garlic and cook an additional 30 seconds. Add flour and stir to coat vegetables. Cook for 1 minute.

Slowly whisk in chicken broth, scraping the bottom of the pan to catch any browned bits.

Add chicken breasts, salt, pepper, and poultry seasoning to the soup and bring to a simmer. Reduce heat to low, cover, and simmer for about 20 minutes, or until chicken breasts are cooked through to 165 degrees Fahrenheit in the center. Remove chicken from soup base and set aside to cool slightly before shredding.

Add milk to soup and stir to combine.

In a bowl, add flour, salt, and baking powder. Whisk to combine and remove lumps. Grate cold butter and add to flour mixture, tossing to combine.

Make a well in the center of your flour mixture and add milk. Gently stir until ingredients are just incorporated. Dough will look shaggy and sticky.

Using a spoon, scoop dough and drop gently into the soup.

Toss dough out onto a floured surface and roll to about ¼" thickness. Slice into 1 inch strips, then cut strips into squares. Drop squares gently into the soup.

Bring soup to a simmer, cover tightly with a lid, and reduce heat to low. Simmer for about 15 minutes, or until dumplings are cooked through to your liking.

Remove lid and add shredded chicken. Stir gently to combine, then remove from heat and serve.