In a large pot, combine the raw chicken breasts and diced onion with chicken broth over medium-high heat. Bring to a boil, then reduce to a simmer and cook for 15-20 minutes until the chicken is fully cooked.

In a separate bowl, whisk together the flour and baking powder. Cut in cold butter until the mixture resembles coarse crumbs.

Gradually stir in the milk until a dough forms, mixing gently until just combined.

Flour a work surface, roll out dumpling dough to 1/4 inch thickness, and cut into 2-inch squares.

Return the broth to a gentle boil and add the dumplings. Cook for 10-15 minutes until they double in size and are firm to the touch.

Add the shredded chicken back into the pot, stir gently and simmer for an additional 5 minutes.