Cut the pumpkin into pieces about 1.5 cm (¾ inch) square, then steam for about 15 minutes until tender.
Leave to cool slightly, then purée in a liquidizer or food processor or press through a sieve until smooth. Weigh the purée, then pour into a preserving pan. For every 500 g (1 lb) of purée add 250 g (8 oz) sugar.
Stir in the spices and ginger, then heat gently, stirring from time to time, until the sugar has dissolved. Increase the heat to medium and cook for about 15 minutes, stirring more frequently towards the end of cooking until the fruit mixture has darkened slightly, is thick, creamy and glossy and drops slowly from a wooden spoon. Skim with a draining spoon or stir in butter if needed.
Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.
To serve, this butter can be used in tiny lattice-topped tarts to make baby pumpkin pies.