Position a rack in the lower part of the oven and preheat to 325 degrees . Grease a 9-inch square baking pan. Have a large roasting pan ready and bring a kettle of water to a boil for the water bath.
In a large bowl, whisk together the brown sugar, eggs, egg yolks and salt. In a large saucepan, bring the milk just to a simmer over medium heat. Remove from the heat, add the chocolate and whisk until smooth. Whisking constantly, drizzle the milk mixture into the sugar mixture in a slow, steady stream. Whisk in the vanilla.
Spread 1 side of each bread slice with butter. Cut the bread slices into triangular quarters. Arrange the bread in the prepared pan and pour in the milk mixture. Let stand at room temperature for 10 minutes, pressing down on the bread frequently to submerge.
Place the baking pan in the roasting pan and place in the oven. Add enough boiling water to the roasting pan to reach halfway up the sides of the baking pan. Bake until a knife inserted into the center comes out clean, about 1 1/2 hours. Transfer the pan from the water bath to a rack to cool slightly. Sift the confectioners sugar on top, if using, and sprinkle with the almonds.