Wash leeks assiduously and slice into rondelles. Wash again if necessary after slicing.) Cube potatoes and place both vegetables in a pot, covering with the broth. Simmer until tender, then purée with an immersion blender. Add salt and white pepper and grated truffle. If you like an exceptionally creamy soup, you can stir in heavy cream at the end, or stir in a knob of butter, or drizzle with a little extra-fine olive oil. Top with a token quantity of finely chopped chives (not too many or the flavor of the truffle and that of the leek will be overpowered) and an additional grating of truffle. Note: As an alternative to the chives, you can chop a bit of the white portion of the leek into small bits and sprinkle this on at the end. The coloring will be more subtle, but the crunch of raw leeks will add to the diner’s tactile experience. You can also sauté the chopped leeks until crispy and top with these, especially if serving your vichyssoise cold. This will add another temperature dimension.