Preheat the oven to 350°F (175°C). Line a cookie sheet with parchment paper.
In a small bowl, mix the instant coffee with the milk until dissolved.
In a large bowl, beat the butter and powdered sugar until smooth and creamy. Add the egg yolk and mix until combined.
Add the coffee mixture and beat until evenly mixed.
Add the cornstarch, cocoa powder, baking powder, and flour. Mix just until a soft dough forms.
Roll small pieces of dough into smooth ovals and place on the prepared cookie sheet.
Press one line down the center of each oval with a small knife or scraper to create the coffee-bean shape.
Bake for 10–12 minutes, until set and lightly firm at the edges.
Cool 5 minutes on the pan, then transfer to a rack to cool completely.