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1
Make the yudane: combine 50 g bread flour with 50 ml boiling water in a small bowl, mixing vigorously until a cohesive paste forms with no dry flour remaining. Cover and refrigerate for a minimum of 8 hours or overnight. The paste will firm and turn slightly translucent — this is the moisture-retention base for the dough.
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2
Dissolve the sake lees in a small measure of the lukewarm milk, working until no solid lumps remain. Combine the bread flour, yeast, sugar, and salt in the bowl of a stand mixer fitted with a dough hook. Add the beaten egg, the remaining milk, and the sake-lees mixture. Add the cold yudane paste in small pieces. Mix on low speed until a shaggy dough forms, then increase to medium and knead for 5 minutes.
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3
With the mixer running on medium, add the softened butter piece by piece, waiting for each addition to be fully absorbed before adding the next. Continue kneading until the dough is smooth, elastic, and pulls cleanly from the bowl walls — a small piece stretched between the fingers should form a thin, nearly translucent membrane without tearing.
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4
Shape the dough into a smooth ball, place in a lightly oiled bowl, cover, and proof at low temperature (refrigerator, or a cool room no warmer than 15 °C) for an extended first rise of 8–12 hours. This retarded fermentation develops flavour. The dough should approximately double in volume and carry a gentle yeast fermentation note.
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5
Divide the dough into 12 equal portions of approximately 45 g each. Divide the anko into 12 equal portions of approximately 50 g each — the anko-to-bread ratio should approach 1:1 by weight. Ensure the anko is at room temperature throughout; cold filling stiffens the dough during shaping and creates uneven proofing.
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6
This is where the dish lives or dies: flatten each dough portion into a round disc, place one portion of anko at the centre, and gather the dough edges upward around the filling, pleating and pinching firmly until the seam is completely sealed. No gap, no tear, no exposed anko — any breach will dry and burn through the thin bread wall during baking. Roll gently between cupped palms to a smooth sphere and place seam-side down on a lined baking tray.
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7
Score the base of each shaped bun lightly with a blade to allow controlled expansion during baking. Space buns well apart on the tray. Cover loosely and proof at room temperature until noticeably puffed and the dough yields gently but springs back slowly when pressed with a fingertip — approximately 60–90 minutes depending on ambient temperature.
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8
Heat the oven to 180 °C. Brush each bun gently with egg wash, then press a small pinch of sesame or poppy seeds flat into the crown. For sakura anpan, press one rinsed, dried salt-pickled cherry blossom into the centre of the crown in place of seeds. The wash should be even and thin — pooling at the base produces an uneven crust colour.
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9
Bake at 180 °C for 13–16 minutes. Pull the buns when the crust is a uniform pale golden brown and an instant-read thermometer inserted through the base reads 88 °C internal temperature. Do not overbake — Japanese filled breads lose their defining tenderness beyond this point. Transfer immediately to a wire rack.