Mix together the ripe, active sourdough starter with bread flour, sugar and water until completely incorporated. Cover and let rise until doubled in size, bubbly and peaked.

About an hour before the levain is ready, whisk together 50 grams all-purpose flour and 250 grams milk in a saucepan. Place the saucepan over medium heat and cook, whisking constantly, until the mixture starts to thicken into a paste. This usually takes about 1-2 minutes. Quickly remove the thickened mixture from the heat and let cool before mixing your dough.

Mix the Dough: To the bowl of a stand mixer, add the cooled tangzhong, ripe, bubbly and active levain, milk, granulated sugar, eggs, salt and bread flour. Turn on the mixer and knead for about 5 minutes until the dough is smooth and all the ingredients well incorporated.

Add Butter: Cut the butter into chunks, about 8-12 per stick of butter. Butter should be firm but soft enough to leave a dent when you press your finger into the butter. Turn the mixer on and add chunks of butter into the dough a little at a time. Continue adding chunks of butter until all the butter is added and incorporates into the dough.

Knead until Windowpane: Continue kneading the dough on medium speed for 20-30 minutes (this will depend on your mixer and your dough) until the dough passes the windowpane test. To check for windowpane: Stop the mixer. Take your hands and pull up on a portion of the dough. At the beginning stages of kneading the dough, the dough will break as soon as you start to stretch it. As the dough kneads and the gluten strands develop, the dough will stretch thinly without tearing. You will know the dough is finished kneading when the dough does not tear when pulled and you can see light through the dough, like a windowpane. A word of caution: As you knead the dough, the friction from the mixer can heat up the ingredients in the bowl. It is important that the dough doesn't get too warm or else the butter will begin to melt and separate from the dough. If you notice this beginning to happen, take a break, stick your dough in the refrigerator to cool down and then continue kneading after the dough cools a little bit.

Bulk Fermentation: At this point the dough will be very soft but also cohesive. Dump the dough into a container and cover. Set the dough in a warm, 78-80ºF place for 4 hours. Take the temperature of the dough as needed to make sure the dough temperature stays right around 78ºF. This temperature is the optimal fermentation. If your dough temperature is cooler than 78ºF, it will need to ferment longer than 4 hours.

Overnight Refrigeration (Cold Fermentation): Stick the covered dough in the refrigerator overnight or up to 48 hours.

Shape Loaves: Prepare two 9 by 5 loaf pans. Take the dough out of the refrigerator. Separate the dough into two equal pieces for two loaves of bread. Cut each loaf into 8 equal-sized pieces. Take each piece of dough and pull/pinch up the sides until it forms a ball. Roll the ball on the counter, holding your hand in a cupping shape (see video here) to seal the balls and create tension. Place 8 balls in each loaf pan, nestling them next to each other. See pictures in the post for details.

Proof: Place the shaped loaves in a warm place to rise until doubled in size and light and airy. I proof my dough in a bread proofer and with the temperature set at 80ºF it takes about 6-7 hours for the dough to rise.

Bake: Pre-heat the oven to 375ºF. Whisk together one large egg with a splash of heavy cream or water. Gently brush the egg wash on top of the risen dough. Bake for 40-45 minutes until the loaves are baked through and the internal temperature is 190-195ºF. Let cool completely before slicing. Enjoy!