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Heat the butter in a large saucepan and add the leeks and bay leaf. Cook over a low heat for 8-10 mins or until the leek is really soft, then stir through the potatoes until coated in the butter. Pour over the stock and cream and bring to the simmer, then gently bubble for 10-15 mins until the potatoes are really tender. If freezing at this stage, slightly under-cook the potatoes, then defrost and bring back to a simmer to finish cooking them and continue the recipe.
Add two-thirds of the smoked salmon, stir through and season. Serve the soup in deep bowls with the remaining smoked salmon and snipped chives on the top.