Step 1

Heat butter in a large saucepan or Dutch oven over medium heat until melted. Add leeks, reduce heat to low, and cook, stirring frequently, until very soft but not browned, 10 to 15 minutes.

Step 2

Add stock, potatoes, and bay leaf, and season lightly with salt and pepper. Bring to a boil over high heat, reduce to a gentle simmer, cover, and cook until potatoes are fall-apart tender, about 15 minutes.

Step 3

To Finish With a Ricer (Recommended): Remove potatoes from soup using tongs and transfer to a bowl. Set aside. Discard bay leaf. Transfer remaining soup to a blender. Slowly increase blender speed to high and blend until completely smooth, about 2 minutes. Return soup to a clean pot.

Step 4

Press potatoes through a potato ricer or food mill into the pot with the soup. Whisk in buttermilk and heavy cream. Whisking frequently, bring soup to a simmer over medium-high heat. Whisk in grated nutmeg. Season to taste with salt and pepper and serve with chives or scallions.

Step 5

To Finish With a Blender (Faster): Add heavy cream and buttermilk to pot. Discard bay leaf. Working in batches if necessary, transfer soup to a blender. Slowly increase blender speed to high and blend until completely smooth, about 2 minutes. Return soup to a clean pot, pressing it through a fine mesh strainer or chinois with the bottom of a ladle if a smoother texture is desired. Whisking frequently, bring soup to a simmer over medium-high heat. Alternatively, chill completely and serve cold. Whisk in grated nutmeg. Season to taste with salt and pepper and serve with chives or scallions.