Assemble the Bread Pudding

Pour half of the custard into the pan. Arrange the bread slices, standing upright, in slightly overlapping rows in the prepared pan. The arranged slices will fill the pan only about two-thirds full. Pour the remaining chocolate custard evenly over the bread slices. It will come almost to the top of the pan and as the bread absorbs the custard, the bread will expand and fill the pan more. Evenly space the rows of slices to fill the pan. Cover the pan lightly with plastic wrap, which will help to keep the slices from floating. Let it stand for 45 minutes while the bread absorbs more of the chocolate custard. If any of the bread slices rise up and float in the mixture during this time, reposition them so that more of the custard can be absorbed.

Bake the Bread Pudding

Place the cake pan in the roasting pan and pour hot water into the roasting pan to come halfway up the sides. Bake for 45 to 55 minutes, or until a thin-bladed knife inserted near the center comes out clean. (An instant-read thermometer should read 170° to 180°F/77° to 82°C.)

Cool and Unmold the Bread Pudding

Remove the pan from the water bath and set it on a wire rack to cool completely, about 2 hours.

Run a small metal spatula between the sides of the pan and the bread pudding, pressing firmly against the pan with the spatula. Place a flat plate, covered with plastic wrap that has been lightly coated with nonstick cooking spray, on top of the pan and invert it. Reinvert the pan onto a cutting board. If the bread pudding does not release, use a small torch or wipe the sides and bottom of the pan with a dish towel that has been run under hot water and wrung out.

After the bread pudding has cooled completely, it can be wrapped airtight and refrigerated for up to 5 days before composing and reheating. The bread pudding can be sliced when it is cold, but should be allowed to warm up to room temperature before reheating.

Preheat the Oven

Twenty minutes or longer before baking, set an oven rack in the middle of the oven and preheat the oven to 350°F/175°C.

Portion the Bread Pudding

Slice the bread pudding into 8 to 10 portions. Cut each portion into 3 slices and arrange them on the baking sheet. If desired, drizzle each serving with about 1 teaspoon/5 ml of bourbon. Loosely cover the slices with aluminum foil and heat for about 10 minutes, or until thoroughly warm.

With a pancake turner, lift the slices and arrange them decoratively on serving plates. Grate bittersweet chocolate lavishly on top.