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Whisk together water, baking soda, cinnamon, and melted chocolate.
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Add the chocolate enrichment to the stand mixer bowl with the dough. Turn mixer on medium; add the 3 tablespoons of flour. Mix until dough is smooth, elastic, and pulls away from the sides of the bowl. Remove to floured work surface.
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Press one of the pieces of plain dough into a 5-inch square on the floured work surface. Repeat with the chocolate dough, placing it on top of the plain dough. Repeat with the last piece of plain dough. It doesn't need to be perfectly lined up.
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Use a knife or bench scraper to cut the dough into 3 strips, each about 1¾ inches wide. Cut each strip into ten ½-inch pieces of bowl. Place the cut pieces of dough into a bowl.
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Slide your hands, palms upward, under the pieces of dough and toss them gently.
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Sprinkle the pieces of dough in the bowl with 1 teaspoon of water. Gently press and squeeze the dough together into a ball. Move dough ball to your floured work surface and press into a thick 4 x 8-inch rectangle. Place the dough in the prepared pan and press to fill the pan completely.
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Cover the pan and let the dough rise about 1 inch over the top of the pan, 1-2 hours.
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Preheat your oven to 350F about 20 minutes before the loaf is fully risen. Place a rack in the middle of the oven.
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Bake until the loaf is well risen and deep golden brown, 35-40 minutes. Cool in the pan for 5 minutes before inverting onto a wire cooling rack.