01 - In a large mixing bowl, combine the all-purpose flour, granulated sugar, active dry yeast, and salt. Whisk together to ensure even distribution.
02 - Add the lukewarm whole milk, lukewarm water, room temperature large eggs, and softened unsalted butter to the bowl with the dry ingredients. Mix until a shaggy, sticky dough begins to form.
03 - Turn the dough out onto a lightly floured surface. Knead for 8 to 10 minutes, or until the dough becomes smooth, elastic, and no longer overly sticky.
04 - Lightly grease a clean bowl with cooking spray or a small amount of oil. Place the kneaded dough into the greased bowl, turning to coat. Cover the bowl with plastic wrap or a clean kitchen towel. Allow the dough to rise in a warm, draft-free place for 1 to 1.5 hours, or until it has doubled in size.
05 - While the dough is rising, prepare the chocolate cinnamon filling. In a small bowl, combine the finely chopped dark chocolate (or chips), packed light brown sugar, unsweetened cocoa powder, and ground cinnamon. Mix until well combined.
06 - Once the dough has doubled, gently punch it down to release the accumulated gases. Turn the dough out onto a lightly floured surface and roll it into a rectangle approximately 9 inches by 18 inches (approximately 23 cm by 46 cm).
07 - Evenly brush the entire surface of the rolled-out dough with the melted unsalted butter. Sprinkle the prepared chocolate cinnamon filling mixture uniformly over the buttered surface, leaving a small border on one of the long edges.
08 - Starting from one of the short ends, tightly roll the dough into a log. Ensure the filling is enclosed within the roll.
09 - Grease a 9x5-inch (approximately 23 cm by 13 cm) loaf pan. Place the rolled dough log into the prepared pan, seam-side down. Cover the pan with plastic wrap or a kitchen towel and let it rise for an additional 30 to 40 minutes, or until it has visibly puffed up.
10 - While the loaf is undergoing its second rise, preheat your oven to 350°F (175°C).
11 - In a small bowl, whisk together the large egg and 1 tablespoon of milk to create the egg wash. Gently brush this mixture over the top surface of the risen loaf in the pan.
12 - Place the loaf pan into the preheated oven. Bake for 35 to 40 minutes, or until the top of the bread is a deep golden brown and sounds hollow when gently tapped.
13 - Once baked, carefully remove the loaf pan from the oven. Allow the bread to cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely. Slice and serve once fully cooled.