Preheat oven to 340℉/170℃ and line a baking sheet with parchment paper.
In a small bowl, stir instant coffee with hot water until dissolved.
In a medium bowl, combine cake flour, cocoa powder, cornstarch, and salt until well combined.
In a large bowl, cream together butter and sugar. Whisk in the egg yolk and coffee mixture until well combined.
Add the dry ingredients to the wet ingredients. Stir with a spatula until well incorporated. The dough will look crumbly at first, but it will eventually come together after some mixing.
Scoop out 8 gr or a teaspoon of dough. Roll the dough into balls, then gently shape them into an oval. Arrange them on the prepared baking sheet. Press the middle of each ball with a knife.
Bake for 15-17 minutes. Cool on a wire rack completely before transferring them to an airtight container.
Notes
Make sure to measure the flour and sugar correctly if using volume measurement. The cookies will end up too crumbly or hard if you add too much flour. Use a spoon to scoop the flour into the measuring cup. Then, level the top with a knife or other straight-edged utensil.Scoop the dough with a measuring spoon instead of a regular teaspoon. Make sure to level the top of the spoon so that each ball comes out in same-size balls.