Mix hot milk with instant coffee until dissolved.

Cream the softened unsalted butter until smooth.

Add powdered sugar and egg yolk to the butter and mix until smooth.

Add the dissolved coffee mixture and mix well.

Sift cake flour, cocoa powder, and corn starch, then fold into the butter mixture using a spatula.

Form the dough into a single ball.

Divide the dough into 5g portions and shape into oval coffee bean forms.

Make a slit in the center of each cookie and chill in the refrigerator for 1 hour.