Place the raisins in a bowl and add the rum. Set aside to plump for 20 minutes, then drain.
Cut the bread into ½-in/1-cm cubes. Place on a sheet pan and drizzle with the melted butter. Toast in a 350°F/177°C oven, stirring once or twice, until golden brown.
Combine the milk and 3 oz/90 g of the sugar in a nonreactive saucepan and bring to a boil.