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1
Beat the eggs with the wasanbon until the mixture is pale, voluminous, and holds a ribbon that dissolves slowly back into itself. The foam is the sole leavening agent; it must be fully developed before any other ingredient is added. The test: the ribbon holds on the surface for three to four seconds before sinking.
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2
Add the honey and mizuame to the egg foam and incorporate evenly.
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3
Add the sifted cake flour and stir the batter with a wooden paddle, always in the same direction. Do not fold. The stirring is deliberate: it develops minimal but consistent gluten, producing the tightly textured, moist crumb that distinguishes castella from génoise. This is where the dish lives or dies — too little stirring leaves an uneven crumb; too much toughens it. The batter is ready when it is smooth, slightly elastic, and pulls away from the paddle in a slow, continuous sheet.
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4
Pour the batter into a wooden box mould lined with paper. The wooden construction insulates the batter during baking, moderating heat transfer and ensuring the even surface colour that defines the finished loaf. Bake until the top is a deep, even golden-brown with no pale patches and no dark spots.
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5
Remove the castella from the oven and invert it immediately onto a board. Place a second weighted board on top and press overnight. The pressing consolidates the crumb into the dense, fine, uniform matrix of a correctly made Nagasaki castella, and produces the smooth, flat surfaces that define the finished product. The test: when the pressed loaf is cut, the cross-section shows fine, uniform, visible cells — not the open, irregular structure of génoise.
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6
Touch the base of the finished loaf lightly with the tongue. A correctly made castella adheres slightly — a faint stickiness from the mizuame. This is the final test that distinguishes a properly made Nagasaki castella from every imitation.