Combine the hot water and coffee together. Mix well and set aside.
In another bowl, combine the flour, cocoa powder and corn starch. Mix well and set aside.
Next, take a bigger bowl, add the butter, sugar and vanilla. Whisk until smooth. Add the egg yolk and continue whisking. Add the coffee mixture to it and whisk.
Fold in the flour mixture. It should form a smooth dough. If not, you may have to add in a few more tablespoons of flour (only if needed). Cling wrap the bowl and refrigerate for 30 mins.
Once chilled, take small portions of the dough and form into coffee bean-like shapes (elongated ovals - see photo on recipe). Place the cookies on a baking tray lined with parchment paper and freeze for 5 mins.
Bake the cookies in a preheated oven at 160°C for 10 to 15 mins.