Disjoint the hen as though it were a frying chicken. Place it in a large pot with the onion, bay leaf, water to cover by 2 inches and salt and pepper to taste. Bring to the boil; reduce heat immediately to low and simmer until the bird is tender. Remove chicken. Cut the meat from the bones in large pieces and reserve. Strain the broth and return 6 cups to the pot.
Now sift flour, 1/2 teaspoon salt and baking powder together into a bowl. Combine egg, melted butter and milk; beat together into a bowl. Combine egg, melted butter and milk; beat together and add to dry ingredients. Work to a good dough like consistency, turn out on a floured board and roll to somewhat less than 1/4-inch thickness. Spread with softened butter; sprinkle with the celery leaves and pepper. Cut into strips 2 x 3 inches.
Bring broth to a good simmering heat; do not boil. Stir in 2 tablespoons flour rubbed with 2 tablespoons extra butter. Cook, stirring, until smooth and thickened. Add the dumplings a few at a time, sliding them down into the hot broth. Continue until all are in. Cover tightly and cook 20 minutes without lifting the lid. Arrange the cut-up chicken on a deep platter, spoon on the dumplings and cover with as much of the delectable sauce as you desire.