Vichyssoise

Melt butter in a large saucepan over medium-low heat; add leeks and season with salt. Cook, stirring occasionally, until leeks are very soft, 8 to 10 minutes. Stir in potatoes, broth, and 1 cup water; season with salt. Bring to a boil, then reduce heat and simmer until potatoes are very tender, 20 to 25 minutes.

Working in batches, transfer mixture to a blender and purée until smooth, adding more liquid as needed. Transfer to a large bowl; whisk in 1 cup cream, milk, and nutmeg; season with salt.

Cover and refrigerate until cold, at least 4 hours and up to 2 days. Stir in remaining 1/2 cup cream, then ladle into bowls and serve drizzled with more cream. Sprinkle with chives and pepper before serving.