Melt half of the butter in a large heavy-based saucepan over a gentle heat but do not allow it to colour. Add the parsnips and leek and let them fry gently for about 4-5 minutes, or until the parsnip is soft.
Add the white wine and chestnuts (reserving a few for garnish) and fry gently for a further 7-8 minutes. Season the mixture with salt and pepper to taste.
Add the stock to the saucepan and bring to the boil, then simmer for 5-6 minutes. Remove the pan from the heat and blend with an immersion blender or in a food-processor until it’s nice and smooth. (If you’re using a food processor, allow the soup to cool a little before you blend it.) Pass the soup through a fine sieve and set aside.
Heat a large frying pan and add the butter. Once melted, add the scallops and cook on each side for 1-2 minutes.
Divide the seared scallops among four shallow serving bowls and ladle the soup around. Grate the reserved chestnuts on top.