Cut the bread lengthwise into 1-inch-thick strips, cutting as far down through the loaf as you can without cutting all the way through. Rotate the bread 90 degrees and cut 1-inch strips in the other direction, without cutting all the way through. Put the loaf into a 1 1/2-quart oval baking dish.
Whisk the milk, brown sugar, cinnamon and eggs in a medium bowl until smooth. Pour the egg mixture over the bread, making sure it gets in all the crevices. Stuff the chocolate pieces in between the rows. Cover and refrigerate for 6 hours and up to overnight.
Preheat the oven to 325 degrees F.
Uncover the bread and drizzle with the butter. Put the baking dish in a larger baking dish and pour hot water into the larger dish until it comes 1 inch up the sides of the smaller dish. Bake until the eggs mixture is completely cooked and the top of the bread is dark golden brown and crispy, 60 to 65 minutes. Remove from the oven and sprinkle with confectioners' sugar. Serve with maple syrup on the side for dipping.
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