Prepare the Levain: Combine 70 g bread flour, 70 g water, and 20 g sourdough starter in a small jar, cover it, and leave it at room temperature overnight until the mixture has doubled in size and looks bubbly.
Cook the Tangzhong: Whisk 48 g bread flour and 240 g whole milk together in a small saucepan until smooth, then cook over low-medium heat while stirring until the mixture thickens into a smooth paste.
Cool the Tangzhong: Scrape the cooked tangzhong into the mixer bowl and allow it to cool before adding the remaining dough ingredients.
Form the Dough: Add 600 g bread flour, 200 g cold whole milk, 70 g granulated sugar, 24 g milk powder if using, 2 large eggs, 12 g salt, and all of the ripe levain to the cooled tangzhong, then mix until a shaggy dough forms.
Knead the Dough: Increase the mixer speed and knead the dough for about 5 minutes, continuing until it becomes smoother.
Add the Butter: Add the 70 g room-temperature unsalted butter in small pieces, then knead for approximately 12–15 minutes until the dough becomes glossy and elastic and begins pulling away from the sides of the bowl.
Check Dough Strength: Let the dough rest for 5 minutes, then stretch a small piece very thin; it should stretch without tearing, indicating that the gluten has developed sufficiently for the windowpane test.
Bulk Rise: Transfer the dough to a lightly oiled bowl and let it rise in a warm place at about 24°C to 26°C / 75°F to 79°F until it has increased in volume by approximately half and feels lighter and puffier, which can take about 3–5 hours depending on the temperature.
Create a Warm Proofing Space: If the room is cold, place the covered dough in a switched-off oven with a mug of boiled water to create a warm, humid environment, replacing the mug when it cools while ensuring the space never becomes hot enough to melt the butter.
Chill the Dough: Refrigerate the dough for about 20 minutes to make it easier to handle and shape.
Prepare the Loaf Pans: Grease or line two 9×5-inch loaf pans in preparation for the shaped dough.
Divide the Dough: Deflate the dough and transfer it to a floured work surface, weigh it, and divide it into 6 equal pieces.
Rest the Dough Pieces: Shape each portion into a smooth ball and let the six balls rest for 10 minutes.
Roll the Dough: Roll each ball into a long oval approximately 30 cm / 11 inches long.
Shape the Logs: Fold the long sides of each oval toward the center, then roll it up from the short end into a neat log and pinch the seam closed.
Fill the Pans: Place the shaped logs seam-side down in the prepared loaf pans, arranging 3 logs in each pan.
Final Rise: Let the shaped loaves rise somewhere warm at approximately 24°C to 26°C / 75°F to 79°F until they become puffy, soft, full, and doubled in size, with the pieces touching; this can take about 3–5 hours depending on temperature.
Preheat the Oven: Heat the oven to 180°C / 350°F once the loaves are ready to bake.
Prepare the Egg Wash: Whisk together 1 large egg and 1 tablespoon water until combined.
Brush the Loaves: Use a pastry brush to coat the tops of both risen loaves evenly with the prepared egg wash.
Bake the Bread: Bake the loaves at 180°C / 350°F for about 35 minutes, until they are deeply golden and sound hollow when tapped.
Cool the Loaves: Remove the baked loaves from their pans and allow them to cool completely before slicing.
