Melt butter in soup kettle and sauté carrots and onions until tender. Sprinkle with flour and stir until blended. Slowly add stock, stirring constantly until smooth. Bring to a boil, reduce heat, and simmer, partially covered, 10 minutes. Beat together cream cheese, yogurt, milk, and egg yolks. Remove soup from heat, cool 10 minutes, and stir in cheese and egg mixture. Reheat, stirring constantly until mixture is smooth. Do not boil. Season with salt and pepper. Garnish with chopped chives.