1-Preheat oven to 340°F (170°C) and line: Preheat oven to 340°F (170°C) and line a baking sheet with parchment paper.
2-In a small bowl, stir instant coffee: In a small bowl, stir instant coffee with hot water until dissolved.
3-In a medium bowl, combine cake flour,: In a medium bowl, combine cake flour, cocoa powder, cornstarch, and salt until well combined.
4-In a large bowl, cream together butter: In a large bowl, cream together butter and sugar. Whisk in the egg yolk and coffee mixture until well combined.
5-Add the dry ingredients to the wet: 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.
6-Scoop out 8 grams or a teaspoon: Scoop out 8 grams 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.
7-Bake for 15-17 minutes: Bake for 15-17 minutes. Cool on a wire rack completely before transferring them to an airtight container.