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1
Make the tangzhong. Whisk 25 g flour with 125 ml cold water in a small saucepan until no lumps remain. Set over medium heat and stir constantly until the mixture thickens and registers 65 °C on an instant-read thermometer — it will turn from opaque slurry to a translucent, cohesive paste that holds a line drawn through it with the spatula. Remove from the heat and press clingfilm directly onto the surface. Cool to room temperature before use.
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2
Combine the flour, sugar, salt, and yeast in the bowl of a stand mixer fitted with the dough hook. Add the warm milk, beaten egg, and the cooled tangzhong. Mix on low speed until a shaggy dough forms and no dry flour remains — the mass will look rough and uneven at this stage.
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3
This is where the loaf lives or dies. Increase to medium speed and knead for 10 minutes, then add the softened butter one cube at a time, waiting for each piece to be fully absorbed before adding the next. Continue kneading at medium-high speed for a further 10–15 minutes until the dough is smooth, glossy, and elastic. Take a small piece and stretch it slowly between your fingers: it must stretch to a near-translucent membrane — the windowpane — without tearing. Any tear means the gluten is insufficient; continue kneading and re-test.
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4
Shape the dough into a ball, place in a lightly oiled bowl, and cover. Allow to bulk-ferment at room temperature until doubled in volume. Press two fingers 2 cm into the surface: the indentation should hold its shape and not spring back immediately.
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5
Turn the dough onto an unfloured surface. Divide into three equal portions. Flatten each portion into a rectangle, roll it up tightly into a short cylinder, then place each cylinder seam-side down in a lightly buttered lidded Pullman tin (kakushiki). The rolls should sit snugly side by side, filling the tin evenly.
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6
Prove, uncovered, until the dough crowns just below the rim of the tin — it should reach within 1 cm of the top edge. Close the lid. The dough must not overflow into the lid mechanism: if over-proofed, the structure will collapse in the oven; if under-proofed, the loaf will be dense. Judge by eye: a gentle dome at lid level is correct.
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7
Slide the closed, lidded tin into an oven preheated to 190 °C (fan 170 °C). Do not open the lid during baking. Bake for 30–35 minutes. The loaf is done when it has pulled slightly from the sides of the tin and produces a hollow sound when the base is tapped through the tin.
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8
Remove the lid immediately on leaving the oven and turn the loaf onto a wire rack. Cool completely — a minimum of one hour — before slicing. The crumb is still setting as the loaf cools; cutting while warm produces a gummy, compressed interior.
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9
To serve as toast, slice at 2.5 cm (the Japanese four-slice standard). Place under a grill at 200 °C for 3 minutes: the surface should be golden and caramelised while the interior remains molten and soft — the crust yields audibly when pressed at the edge.