In a large bowl, whisk the softened unsalted butter until creamy.
Add the icing sugar to the butter and whisk until smooth and free of lumps.
Incorporate the egg yolk into the mixture and mix until well combined.
Dissolve the instant coffee powder in hot water, then add it to the yolk mixture and stir until fully combined.
In a separate bowl, combine all-purpose flour, cocoa powder, and corn starch.
Gradually sift the dry ingredients into the wet mixture, one-third at a time, mixing until a soft dough forms.
Divide the dough into 8g portions and shape each into an oval.
Using the back of a butter knife or a toothpick, gently press a line down the center of each dough piece to resemble a coffee bean.
Place the shaped dough onto parchment paper in a container, cover, and freeze for 30 minutes.
Preheat the oven to 170°C (325°F).
Transfer the chilled dough onto a baking sheet lined with parchment paper.
Bake for 12-14 minutes, until the cookies are set.
Carefully transfer the cookies to a cooling rack and allow them to cool completely before serving.