2) Raise the heat to high, pour in the vermouth, and cook, uncovered, until almost dry, about four minutes.
3) Add the potatoes, water and salt. Tie the parsley, thyme and bay leaf together with a piece of kitchen twine and add to the soup. Bring to a boil, lower the heat, and simmer, covered, until the potatoes are very tender, about 20 minutes. Remove from the heat and allow to cool.
4) When the potato mixture has cooled, remove and discard the herb bundle. Working in batches, transfer the mixture to a blender and puree until smooth, or puree with an immersion blender.
5) Return the puree to the pot and re-heat over medium heat. Whisk the reserved 30g butter, double cream and lemon juice into the soup and season with pepper, to taste. Divide among warm soup bowls, garnish with the chives, and serve immediately.