In a large deep heavy bottom pot, like a dutch oven, add the chicken, carrots, celery, onion, garlic, thyme, water, vinegar, bouillon, pepper, and bay leaves. Bring to a simmer over medium-high heat and cook for 30 minutes.
While the soup is cooking, combine the flour, baking powder, and salt in a medium sized bowl. Use a fork and cut in the shortening until small pea sizes remain, then stir in the buttermilk. Mix it all together and knead for 30 seconds.
Dust a flat surface with flour and roll out the dough to about 0.25 inch thickness. (Add extra flour if your dough is sticking while rolling it out.)
Use a knife or pizza cutter to slice the dough into 1” by 2” rectangle pieces (does not need to be exact in size). Let the dough dry while the soup continues to cook.
When the chicken is cooked, remove it from the soup and shred it. Remove the thyme sprigs and discard. Stir the chicken back into the soup.
Bring soup back to a low boil (over medium heat) and drop the dumpling pieces into the soup a few at a time, stirring often so they don’t all stick together. Cook for 15 to 20 minutes, until they are tender and cooked through. Dumplings may start to sink a little to the bottom as they finish cooking as they soak up the broth.
Thicken If Needed: The soup will have thickened while adding the dumplings, however if the broth is too thin for your tastes, combine 3 tablespoons of water and the cornstarch and pour into the broth, gently stirring to combine. Cook for about 5 minutes, until thickened.
Remove from the heat, taste and adjust salt and pepper if needed and serve immediately!