Heat 2 1/2 tablespoons of the butter in a heavy soup kettle. Chop the spinach and the white part of the leeks, and cook gently in the melted butter. When softened, sprinkle in flour, cook for 2 minutes, at medium heat, stirring constantly. Remove kettle from heat and add broth, water and salt. Return to low heat and stir until the mixture thickens slightly. Simmer for 15 minutes. Mash the cream cheese in a small bowl. Sprinkle with salt and pepper to taste and stir in the yogurt and egg yolks. Beat mixture until smooth. Sauté ham in remaining butter. Carefully stir the cheese mixture into the soup and cook over low heat for 5 minutes, stirring constantly. Add the ham. Serve the soup piping hot, garnished with chopped chives. Serves 8 people.