Bring the butter to room temperature. Sift the okara powder and cake flour together.

In a heatproof bowl, mix the butter and sugar until pale and creamy. Gradually add the beaten egg in three parts, mixing well each time.

Add one-third of the sifted dry ingredients to the wet mixture and mix until fully incorporated. Gradually add the remaining dry ingredients and mix gently until the dough comes together. Knead lightly by hand to form a smooth dough.

Place the dough on plastic wrap, shape it into a cylinder about 3cm in diameter, and refrigerate for 2 hours.

Slice the chilled dough into 7mm thick rounds and arrange them on a baking sheet lined with parchment paper.