Step 1: Make the Tangzhong

1. In a small saucepan, whisk together the 20g bread flour and 100g water (or milk) until no lumps remain.
2. Place the saucepan over medium-low heat. Cook, whisking constantly, until the mixture thickens into a smooth paste, resembling mashed potatoes. Use a thermometer to check for 65°C (149°F) if possible. This should take 2-3 minutes.
3. Remove from heat immediately. Transfer the Tangzhong to a small bowl. Cover the surface directly with plastic wrap (to prevent a skin from forming) and let it cool completely to room temperature. You can speed this up in the refrigerator, but bring it back to room temp before adding to the dough.

Step 2: Prepare the Coconut Filling

1. In a medium bowl, cream the softened 50g butter and 60g granulated sugar together using a spatula or electric mixer until light and fluffy.
2. Gradually beat in the 1 beaten egg until fully incorporated.
3. Stir in the 30g milk powder and 20g cake flour (or AP flour) until combined.
4. Fold in the 100g desiccated coconut until evenly distributed.
5. Cover the bowl and refrigerate the filling for at least 30 minutes (or until ready to use). This helps it firm up, making it easier to handle.

Step 3: Make the Main Dough (Stand Mixer Instructions)

1. In the bowl of your stand mixer, combine the 350g bread flour, 50g sugar, 5g salt, 5g instant yeast, and optional 20g milk powder. Whisk briefly to combine.
2. Add the cooled Tangzhong, 1 beaten egg, and about 110g of the lukewarm milk.
3. Using the dough hook attachment, mix on low speed until a shaggy dough forms.
4. Increase the speed to medium-low (e.g., speed 2-4 on a KitchenAid) and knead for about 8-10 minutes. The dough will start rough but should become smoother and more elastic. If it seems excessively dry, add the remaining 10g milk cautiously; if too sticky, add a tiny bit of flour (1 tsp at a time).
5. Add the 40g softened butter, one piece at a time, mixing on low speed until incorporated.
6. Increase the speed back to medium-low and continue kneading for another 10-15 minutes, or until the dough becomes very smooth, elastic, and passes the “windowpane test.” (To check: pinch off a small piece of dough and gently stretch it thin. You should be able to stretch it thin enough to see light through it without it tearing easily). This gluten development is crucial for soft buns.

*(Hand Kneading Alternative: Combine dry ingredients in a large bowl. Make a well, add wet ingredients (Tangzhong, egg, milk). Mix to form a shaggy dough. Turn out onto a lightly floured surface. Knead for 10-15 mins until smoother. Gradually incorporate softened butter and continue kneading vigorously for another 15-25 mins until smooth, elastic, and windowpane stage is reached. It’s a workout!) *

Step 4: First Proof (Bulk Fermentation)

1. Lightly grease a large bowl with oil or cooking spray.
2. Shape the kneaded dough into a smooth ball. Place it in the greased bowl, turning it once to coat the surface lightly with oil.
3. Cover the bowl tightly with plastic wrap or a damp cloth.
4. Let the dough rise in a warm, draft-free place for 60-90 minutes, or until doubled in size. The exact time depends on room temperature and yeast activity.

Step 5: Prepare the Topping Streusel (If Using)

1. While the dough is proofing, prepare the streusel if you’re using it.
2. In a small bowl, combine the 25g cold cubed butter, 20g powdered sugar, and 35g all-purpose flour.
3. Use your fingertips or a pastry blender to rub the butter into the dry ingredients until the mixture resembles coarse crumbs. Don’t overwork it.
4. Chill the streusel mixture in the refrigerator until needed.

Step 6: Divide and Shape the Dough

1. Once the dough has doubled, gently punch it down to release the air.
2. Turn the dough out onto a very lightly floured surface.
3. Divide the dough into equal portions. For standard-sized buns, divide into 10 or 12 pieces (each weighing around 60-70g). Use a kitchen scale for accuracy.
4. Gently shape each portion into a smooth ball (pre-shaping).
5. Cover the dough balls loosely with plastic wrap and let them rest for 15 minutes. This relaxes the gluten, making them easier to shape.

Step 7: Fill and Form the Buns

1. Line one or two large baking sheets with parchment paper.
2. Take one dough ball. On a lightly floured surface, gently roll it out into an oval shape, about 4-5 inches long.
3. Retrieve the chilled coconut filling from the refrigerator. Divide the filling equally among the dough portions (use a scale or eyeball it).
4. Place one portion of the filling down the center of the rolled-out oval, leaving a border around the edges. Shape the filling into a log.
5. Fold one long side of the dough over the filling. Fold the other long side over, overlapping slightly, like folding a letter. Press the seam gently to seal. You should have a log shape.
6. You can leave them as logs or gently curve them into a crescent shape if desired. Place the shaped bun, seam-side down, onto the prepared baking sheet.
7. Repeat with the remaining dough balls and filling, placing the buns about 2 inches apart on the baking sheet(s).

Step 8: Second Proof

1. Loosely cover the shaped buns on the baking sheets with plastic wrap (grease the underside lightly if worried about sticking) or a lightweight cloth.
2. Let the buns rise in a warm place for another 45-60 minutes, or until they look noticeably puffy and almost doubled in size. When gently poked, the indentation should spring back slowly. Don’t overproof.
3. About 15-20 minutes before the end of the proofing time, preheat your oven to 350°F (175°C). Position a rack in the center of the oven.

Step 9: Apply Egg Wash and Topping

1. In a small bowl, whisk together the 1 egg and 1 tablespoon of water or milk for the egg wash.
2. Gently brush the tops and sides of the proofed buns with the egg wash. Be careful not to deflate them.
3. If using streusel: Sprinkle the chilled streusel topping evenly over the buns.
4. If making classic lines: You can pipe thin lines of a simple paste made from flour, butter, and sugar across the top, or simply leave them plain with egg wash. Some bakeries add sesame seeds or almond slivers after the egg wash.

Step 10: Bake

1. Place the baking sheet(s) into the preheated 350°F (175°C) oven.
2. Bake for 15-20 minutes, or until the buns are beautifully golden brown on top and cooked through. If you have an instant-read thermometer, the internal temperature should reach around 190-200°F (88-93°C).
3. If the tops are browning too quickly, you can loosely tent the buns with aluminum foil for the last few minutes of baking. Rotate the baking sheet halfway through if your oven bakes unevenly.

Step 11: Cool

1. Carefully remove the baking sheets from the oven.
2. Let the buns cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
3. Optional Finishing Touch: While the buns are still warm, you can brush them lightly with a simple syrup (equal parts sugar and water, heated until dissolved and cooled) or a thin honey glaze (honey warmed slightly with a splash of water) for extra shine and a touch of sweetness.