1. Preheat the oven to 350°. Place the wood chips in the bottom of a large stockpot and set over high heat. Once the wood chips become fragrant, after about 5 minutes, place a steamer insert over the chips and set the potatoes in the steamer. Cover the pot and transfer to the oven. Cook until a paring knife easily slips into the center of the potatoes, about 1½ hours. Remove from the oven and set aside to cool slightly, then peel the potatoes.

2. Meanwhile, make the soup base: In a stockpot set over medium heat, melt the butter. Add the onion, leek and celery and cook, stirring often, until the vegetables are tender, about 5 minutes. Pour in the water, cream and milk, and then stir in the miso, salt and pepper. Turn the heat to high and bring the mixture to almost a boil, then reduce the heat to medium-low and simmer gently for 20 minutes. Turn off the heat and set aside to cool for 10 minutes.

3. Add the potatoes, buttermilk and sherry vinegar and use an immersion blender to blend until smooth (alternatively, blend in a blender, working in batches, then return the soup to the stockpot). Warm gently over medium heat. Serve hot.