For a smooth, cold soup, press through a food mill and chill before serving.
Melt 4 tablespoons butter in heavy skillet. Saute leeks and celery until tender. Scald milk in soup kettle and stir in leeks and celery. Simmer, uncovered, over very low heat 30 minutes. In a separate pan cook potatoes in boiling water 10 minutes and drain. Put remaining 2 tablespoons butter in a saucepan and slowly stir in flour until mixture is smooth. Add ½ cup liquid from soup kettle and stir until thickened. Return mixture to kettle. Add potatoes to kettle, cover, and simmer 15 minutes. Season with salt and pepper. Garnish with bacon or chopped parsley.