In a heavy saucepan, melt butter over med. heat; cook leeks, covered and stirring occasionally, for 10 minutes or until softened, adding up to 2 tbsp. of the chicken stock if mixture seems dry. Add potatoes and remaining chicken stock; bring to boil. Reduce heat to medium; simmer for 20 minutes.

Reserve 1/2 cup of the watercress leaves for a garnish. Add remaining watercress to saucepan; cook for 1 minute or just until wilted. In a blender or food processor, purŽe watercress mixture, in batches, until smooth. Blend in milk. Refrigerate for at least 4 hours or up to 8 hours. Season with salt & pepper to taste. Chop reserved watercress and sprinkle over individual servings. Makes about 4 cups or 4 servings.