In a small saucepan, whisk together 43g bread flour and 215g milk until smooth with no lumps.

Place over medium heat and whisk constantly. The mixture will start thin, then gradually thicken into a paste.

Continue cooking and whisking until the mixture reaches 150°F (65°C) on an instant-read thermometer, about 3-4 minutes. You'll see lines forming in the paste as you whisk.

Remove from heat and transfer to a small bowl. Press plastic wrap directly onto the surface to prevent a skin from forming. Let cool to room temperature (30-45 minutes).

In the bowl of a stand mixer, combine 100g active sourdough starter and 240g milk. Whisk until the starter is fully dissolved and the mixture looks milky.

Add the cooled tangzhong, 500g bread flour, 10g salt, and 30g honey to the bowl.

Using the dough hook attachment, mix on low speed (setting 2) for 2-3 minutes until a shaggy dough forms and no dry flour remains.

Add 40g softened butter to the dough. Continue mixing on low speed until the butter is fully incorporated, about 2 minutes.

Increase speed to medium-low (setting 4) and knead for 8-10 minutes. The dough should become smooth, elastic, and pull away from the sides of the bowl. It will still be slightly sticky at the bottom but should pass the windowpane test.

Transfer the dough to a lightly greased bowl. Cover with plastic wrap or a damp towel.

Let the dough rise at room temperature (68-72°F) for 10-12 hours, or until doubled in size. In warmer kitchens (75-78°F), this may take 8-10 hours.

The dough is ready when it passes the poke test: gently press your finger ½ inch into the dough—if the indent slowly springs back halfway, it's ready.

Grease your 9×5 inch loaf pan with butter or cooking spray.

Turn the dough out onto a lightly floured work surface. Gently press it into a rectangle roughly 8 inches wide and 12 inches long.

You want to degas the dough gently to create an even crumb structure (this is different from artisan loaves where you preserve big bubbles).

Starting from the short end, roll the dough tightly into a log, pinching the seam closed as you go.

Place the shaped dough seam-side down in the prepared loaf pan. Gently press down to fill the corners of the pan.

Cover the pan loosely with plastic wrap or a reusable cover.

Let the dough proof at room temperature for 2-4 hours, or until it crowns about 1 inch above the rim of the pan.

When you gently press the surface, it should feel light and airy and spring back slowly.

Preheat your oven to 375°F (190°C) with the rack in the lower third position.

For a shinier top, brush the dough gently with milk or a beaten egg.

Bake for 35-40 minutes, until the top is deep golden brown. If the top browns too quickly, tent with aluminum foil after 25 minutes.

The bread is done when it reaches an internal temperature of 190-195°F (88-91°C) in the center.

Remove from the oven and immediately brush the top with melted butter for an extra-soft crust (optional but highly recommended).

Let cool in the pan for 5 minutes, then turn out onto a wire rack. Cool completely before slicing (at least 1 hour). Cutting into hot bread will give you a gummy texture.