Melt soup butter in large wide pot; cook onion, carrots, and celery 6 minutes. Stir in soup flour 1 minute; gradually add broth.
Add chicken, soup salt, thyme, and pepper; gently simmer 12 minutes.
Whisk dumpling flour, baking powder, and dumpling salt in bowl. Add milk and melted dumpling butter; stir just until soft, slightly rough dough forms.
Drop dough into simmering soup in 12 small mounds with gaps between. Cover tightly; gently simmer 15 minutes without lifting lid.
Check dumpling center is fluffy, not wet, and 160°F; chicken must reach 165°F. Cover and cook 2 to 3 minutes more if needed. Serve two dumplings per bowl.