Remove the bay leaf. For the lightest texture: remove the cooked potatoes and pass them through a potato ricer (or fine-mesh strainer).
Meanwhile, blend the leek-and-broth mixture (without potatoes) until smooth.
Return both parts to the pot, whisk in the cream and buttermilk, and stir in the nutmeg.
If you prefer a quicker version you may dump everything into a blender and purée, though texture will differ.