The day before:

Make the Yudane:

1. In a bowl, combine 75g bread flour in a bowl and boiling water and mix well with spatula or spoon.
2. Cover tightly and place in the fridge for at least 4 hours or overnight. The mixture will be stiff after mixing, and soft and pliable the next morning. Use directly from the fridge.

Make the black sesame seed paste:

1. Process the black sesame seeds, oil, and honey (if using) in a high speed blender until it reaches a smooth consistency.
2. If the paste is too thick, add a bit more oil to thin it out. If it’s too runny, add more toasted sesame seeds. Note: Adding too much honey will affect the overall flavor of the loaf so use it sparingly. For this bread, you want a nutty flavor.

The next day:

1. The next day, or at least four hours after making the yudane, tear the yudane dough into pieces and combine with the flours, yeast, brown sugar, salt, flax seed meal, and milk in the bowl of a stand mixer. Stir together with a spatula or paddle attachment for 2 minutes to incorporate. Change to the dough hook and knead for another 3 minutes or until the dough comes together.
2. Add the butter and continue kneading for another 10 – 13 minutes or until the gluten is well developed and can be drawn out into a fairly transparent sheet (windowpane test). This dough should be slightly tacky but not sticky. Scrape down the dough from the hook periodically during the kneading process to ensure well distributed gluten development.
3. Using a scale and a bench knife, separate the dough into 1/3 and 2/3 portions. Place the smaller 1/3 portion of dough back in the mixer, add the black sesame paste, and knead until combined. About 3-4 more minutes.
4. Place the plain and sesame dough each in their own bowl. Cover and let rise in a warm place for 1-2 hours or more, until double in size.
5. When the doughs have doubled, turn them out onto a lightly floured surface. Using a rolling pin, roll out the plain dough into an 8×12-in (20×30-cm) rectangle. Roll out the sesame dough into an 8×6-in (20×15-cm) rectangle. Place the sesame dough on top of one side of the plain dough and fold the plain dough over to enclose the sesame dough.
6. Using a bench scraper, pizza cutter, or knife, cut the dough in half width-wise, then stack the two pieces on top of each other. Roll the dough back into an 8 x 12 in (20 x 30 cm) rectangle. Repeat this step twice more, having finally cut and stacked a total of three times.
7. Now, using the bench scraper, pizza cutter, or knife, slice the dough lengthwise into 3 equal 12-inch strips. Roll and twist each strip into a tube/rope shape and braid the ropes together.
8. Grease an 8×4-in loaf pan. Fold the ends of the braided dough under to fit in the bread pan. Proof in a warm place for 45 min to 1 hour or until doubled in size.
9. While the loaf is rising, preheat oven to 350°F (175°C). When the loaf has doubled, brush with egg wash. Bake for 30 minutes until golden and baked through, (internal temperature should be at least 190ºF).
10. Cool in the pan for about 5-10 minutes. Remove the loaf from the pan and finish cooling on a wire rack completely before slicing and serving.