In a dutch oven place chicken breast, 4 cups water, bouillon cubes, carrots, onion, celery, sage, rosemary sprig and garlic. Simmer until chicken is done about 30 minutes.
Remove chicken to cool and discard rosemary sprig. While chicken is cooling assemble dumpling batter in a medium bowl - mix flour, baking powder, and salt. Add milk and stir until combined.
Drop by large tablespoon the dumpling batter into the simmering broth until all batter is used. Skin and debone chicken breast and chop into bite sized pieces and add back to broth. Whisk corn starch into 1/4 cup water to combine. Slowly pour into pot while stirring to thicken. Add pepper to taste.