Heat the milk in a microwave-safe bowl for 20 seconds, then whisk in the instant coffee. Keep whisking until the liquid looks smooth, without visible granules.

Pass the cake flour, cornstarch, and cocoa through a sieve into a large bowl and reserve. The combined dry mixture should look evenly colored and free of cocoa lumps.

Work the softened butter with a whisk in a separate bowl, incorporate the powdered sugar, then whisk in the yolk. Stop when the mixture looks pale and creamy rather than continuing to whip it fluffy.

Blend the dissolved coffee into the butter mixture until no streaks remain. The pale mixture should take on an even, light mocha color.

Sift the reserved dry mixture over the wet mixture, stir to form crumbs, then gather and press everything together by hand. The initially loose crumbs should become a smooth dough that holds together.

Form a long dough log, cut it into roughly ½-inch portions weighing about 8 g each, and roll each portion between your palms into an oval. Each piece should be about 1 teaspoon of dough with a rounded bean shape.

Indent each oval lengthwise with a toothpick or butter knife, arrange on a parchment-lined or cooking-spray-greased baking sheet, and refrigerate for about 1 hour. Press only halfway into the dough so the crease stays deep without splitting the cookie.

Set the oven to 325°F and allow it to preheat. The shaped dough should remain chilled while the oven comes to temperature.

Move the chilled sheet into the preheated oven and bake for 12 to 14 minutes. Look for dry surfaces and set edges rather than waiting for pronounced browning.

Allow the cookies to cool for 10 minutes before handling. The tender cookies firm as they cool, making them less likely to break.