Place a large stockpot or Dutch oven over medium heat, add butter. Once melted add celery and saute for 4 minutes. Add onion and carrot, continue cooking for an additional 3 minutes. Add garlic and saute for 1 minute.
Add flour and whisk until thickened and lightly golden in color. Slowly pour in chicken broth, whisking constantly. Slowly add in milk, whisking constantly. Add salt, pepper, garlic powder and thyme. Bring mixture to a boil, then reduce heat and simmer for 20 minutes.
Meanwhile, to make the dumplings, in a medium mixing bowl, whisk together the flour, baking powder, and salt. Pour in the milk and melted butter, using a spatula fold until a soft, clumpy dough forms. Do not over-mix.
After simmering for 20 minutes, stir in the chicken.
Using a standard or large size cookie scoop, scoop biscuit dough over top of soup. Gently press the dumplings into the liquid, submerging them slightly, then let them pop back up to the surface. Note: some of the dumplings will be overlapping, that's ok.
Cover and cook for 15-20 minutes or until the dumplings are cooked through.