In the pressure cooker, place the chicken, celery, onion, bay leaf, salt, pepper and garlic. ***see notes section about salt and pepper if using bouillon cubes later*** Add 1/2 cup of water and seal the lid. Bring to full pressure on the stove top. (If using an Instant pot, set for 8 minutes at full pressure). Hold at full pressure for 8 minutes and allow a natural release.
After natural release, remove the chicken and the bay leaf (discard bay leaf) and place chicken on cutting board. Using two forks, shred the chicken. Remove any bones, cartilage and skin if you have those and then place the shredded chicken back into the pan. Use a large stock pot or dutch oven if you don't have a pressure cooker that you can place onto the stove (i.e. instant pot).
Add the chicken stock to the pot. It should cover the chicken by a couple of inches. Bring to a boil.
While the stock pot is coming to a boil, make your dumplings. Mix the baking mix and the milk together in a large bowl using a spatula. Dust a large board or your counter with baking mix and turn out onto the board. Knead, adding enough baking mix as needed to keep the dough from sticking to your hands and the board. Knead until soft and pliable. Sprinkle the board well with bisquick flour and roll out into a thin sheet 1/8 inch thick (about). Use a table knife turned upside down to cut the dough into 1-2 inch wide strips.
Once the stock is boiling, add the dough to the pot in strips a few at the time. You'll notice that they sink immediately and then float to the top. Soon the top will be covered with a mat of dumplings. Gently, using a rubber spatula, open a spot in the dumplings to add more until you have added all of your dumplings to the pot. Allow to gently boil for 1-2 minutes and then set the temp on medium low and allow to cook for 10 or so minutes. Use your spatula to spoon the hot soup over the top of the dumplings as they cook. During this time, add the milk and butter.
The chicken and dumplings are ready when the noodles begin to sink and when tasted are done throughout.