Reserve ¾cup of the chicken stock. Place the remainder in a saucepan or casserole and add the watercress stems. Bring to a boil and simmer for about 5 minutes.
Blend the cornstarch with the reserved cold chicken stock and stir it into the boiling broth. Remove from the heat and blend in an electric blender. Add the cream and bring to a boil. Add salt and pepper to taste. Add the lemon juice and remove from the heat.
Swirl in the cold butter and add the watercress leaves. Serve immediately.