Grease six 6cm pudding moulds with melted butter.
Boil sultanas and currants in rum and let stand for 30 minutes.
Mix flour, baking powder, suet, sugar, and lemon zest in a bowl.
Stir in the soaked fruit and milk to form a soft dough.
Fill moulds three-quarters full and secure with pleated foil and string.
Steam moulds in a saucepan with boiling water for 1 hour.
Gently warm cream, milk, and vanilla seeds in a saucepan.
Whisk egg yolks and sugar, then incorporate warm milk and thicken over heat.
Heat crème de cassis, sugar, and blackberries for 5 minutes and strain.
Serve puddings with custard and a drizzle of blackberry sauce.