In a separate pan, melt butter over medium heat. Whisk in flour. Cook, stirring constantly, for 1 to 2 minutes. Whisk in the heavy cream, tarragon, and cilantro. Bring the cream mixture to a boil, and cook, stirring constantly, until thickened. Stir the cream mixture into the potato mixture. Puree about 1/2 the soup, and return to the pan. Adjust seasonings to taste.