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1
Combine 15 g bread flour, 75 ml water, and 75 ml whole milk in a small saucepan. Stir continuously over medium heat until the mixture reaches 65 °C and coats the back of a spoon in a thick, pale paste. Remove from heat. Transfer to a bowl and cool completely before use. The tangzhong is ready when it is opaque, cohesive, and no longer warm to the touch.
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2
Combine bread flour, sugar, salt, and dry yeast in a large bowl, keeping salt and yeast on opposite sides of the bowl. Add the warm milk, heavy cream, beaten egg, and the cooled tangzhong. Mix until a shaggy dough forms, then knead until the mass is smooth and begins to clear the sides of the bowl. Add the softened butter a few cubes at a time, continuing to knead until each addition is fully absorbed. The dough is ready at this stage when it is smooth, supple, and no longer sticks to the hands.
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3
This is where the dish lives or dies. Perform the windowpane test: tear a small piece of dough and stretch it slowly between the fingers into a thin film. The dough must stretch to near-translucency without tearing, the light visible through it in an unbroken membrane. If it tears, knead further and test again at two-minute intervals. An under-developed gluten network cannot hold the high hydration of this dough through shaping or the final proof; no subsequent step can recover the loss.
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4
Shape the dough into a smooth ball, place in a lightly oiled bowl, and cover with a damp cloth. Prove at a warm room temperature until the dough has increased by approximately 50–60 percent in volume — not a full double. Press one finger lightly into the surface; the indentation should spring back slowly but not fully collapse. Deflate gently.
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5
Divide the dough into equal portions, shape each into a smooth log, and arrange in a lightly greased Pullman pan. Slide the lid on, leaving the smallest gap possible. For the yama (mountain) version, use a standard loaf pan without a lid. Prove again at a warm, draught-free spot until the dough has risen 50–60 percent and the surface, pressed lightly, springs back slowly. Do not open the Pullman lid during this second proof.
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6
Slide the closed Pullman pan into an oven preheated to 190 °C. Bake 30–35 minutes. Do not open the lid during baking; steam trapped inside the pan is essential to the square cross-section and to the crumb structure. The loaf is done when a deep, hollow knock on the base of the pan returns a resonant sound and the internal temperature reads 90–95 °C.
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7
Unmould immediately onto a wire rack. The crust should release cleanly from the Pullman sides without resistance. Cool to room temperature before slicing. Shokupan is at its optimum within 6 hours of baking; the crumb softness and mochi-like spring are substantially altered after 24 hours, and refrigeration accelerates staling. Slice at 1–1.5 cm for eating plain; at 2–3 cm, crust removed, for tamago sando.