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Put the salmon fillets in one layer in a microwaveable dish and dot with 25g/1oz of the butter, salt and pepper. Cover with cling film, stabbed several times, then microwave on High for 5-6 minutes, until the salmon is just cooked. (Or put in a frying pan, cover with boiling water, season and poach for 5-6 minutes, and add the butter to the food processor.) Leave to cool.
Flake the salmon into a food processor, adding the juices from the dish. Chop the smoked salmon roughly and add to the processor with the lemon juice, chilli or harissa paste, salt and pepper.
Process until finely chopped, but with a bit of texture, then turn into a 1 litre serving dish. Melt the remaining butter in a small pan, remove from the heat and allow the sediment to settle. Carefully pour the butter over the salmon, leaving the sediment behind. Leave to cool, then chill until set, about 2 hours. (The salmon may be frozen for up to 1 month.)