In a large bowl, whisk the softened unsalted butter until creamy.
Add the icing sugar into the butter and whisk until combined with no lumps.
Add the egg yolk into the mixture. Mix until combined.
Mix the instant coffee powder and hot water until combined. Add into the yolk mixture and mix until combined.
In a separate bowl, combine all-purpose flour, cocoa powder, and corn starch.
Sift the dry ingredients into the batter gradually – one-third at a time. Mix until it forms a soft dough.
Separate the dough into balls of 8g each. Shape them into an oval shape.
Carefully use the back of a butter knife or toothpick to press a middle line on each dough.
Place them on parchment paper and in a container with a lid. Put them into the freezer for 30 minutes.
Preheat the oven at 170 °C or 325 °F.
Transfer the coffee bean cookies on to a baking sheet with parchment paper carefully. Bake for 12-14 minutes.
Carefully place the cookies into the cooling rack. Ready to serve.