Butter an 8-by-11-inch glass or ceramic baking dish. In a medium glass bowl, microwave the chocolate at high power in 30-second intervals until melted, stirring between intervals.
In a medium saucepan, bring the milk and cream to a simmer over moderately high heat. In a large bowl, whisk the whole eggs and yolks with 1/2 cup of the sugar, the vanilla and salt. Slowly whisk the hot milk mixture into the egg mixture, then whisk in the melted chocolate.
Spread the challah cubes in the prepared baking dish and pour the chocolate custard over the top. Press the challah into the custard until evenly soaked, then let stand for 20 minutes.
Preheat the oven to 325°. Sprinkle the remaining 2 tablespoons of sugar over the bread pudding. Set the baking dish in a roasting pan and fill the pan halfway with hot water. Bake the bread pudding for 50 minutes, until a knife inserted into the center comes out clean. Remove the dish from the water bath and let stand for 20 minutes. Serve the bread pudding warm or at room temperature with the Bourbon Caramel Sauce and whipped cream.
