The day before you want to serve this dessert, if possible, put raisins in a small saucepan, cover with water, and bring to a boil. Remove from heat and drain. Place plumped raisins in a plastic container with a tight-fitting cover and sprinkle with the rum. Cover and let macerate overnight.
Preheat oven to 325 degrees. Melt the butter and cool slightly.
Cut the challah in half, and cut one of the halves into thin slices. Cut remaining bread into cubes.
Put diced challah in the bottom of the baking dish. Strew with the raisins and rum and half the butter. Dip one side of each slice of challah into the butter and arrange, slightly overlapping and buttered side up, over the top of diced challah and raisins.
Combine the milk, cream and sugar, and vanilla bean in a saucepan. Bring to a boil over medium heat. Remove from heat, add chocolate, and allow to stand 3 minutes; whisk smooth.
Whisk eggs until combined in a large mixing bowl, then strain the milk mixture into the eggs and beat them together. Do not overbeat or the custard will have a great deal of foam on the surface. Strain the custard back into the pan and use a large spoon to skim any foam from the surface. Pour the custard over the challah.
Place the baking dish in another larger pan and pour warm water into it to come halfway up the side of the baking dish. Bake the bread pudding for about 45 minutes, until the custard is set and the challah is an even color.