Preheat the oven to 220°C (425°F/Gas 7). Peel a butternut pumpkin (about 450 g/1 lb before cooking), remove the seeds and cut into pieces. Place on a baking tray lined with baking paper, add enough water to cover the paper and bake for about 1 hour, until the pumpkin flesh is very soft. Let the pieces of pumpkin cool, then process them until you have a quite smooth purée. If it is too watery, let it drain overnight in the refrigerator in a colander lined with a clean cotton tea towel (dish towel).