First, cream butter and powdered sugar.
⅓ cup salted butter, softened to room temperature, ⅔ cup powdered sugar
Then whisk in the egg yolk, brewed coffee, and vanilla extract.
2 tablespoons freshly brewed coffee, cooled to room temperature, 1 egg yolk, 1 teaspoon vanilla extract
In a separate bowl, mix together the cake flour, cornstarch, and cocoa powder.
1 cup cake flour* (see "notes" below), 3 tablespoons cornstarch, 2 tablespoons cocoa powder
Slowly add dry to wet ingredients. The resulting batter will almost look like cake frosting.
Place the bowl in the fridge for 1 hour to chill, which will allow it to firm up a bit and be much easier to work with and form into coffee bean shapes.
Preheat oven to 325℉.
Using a teaspoon (a 1-teaspoon measuring spoon, not the one from your silverware drawer), measure out heaping teaspoonfuls of batter.
Roll the teaspoonfuls of batter between your hands, forming a ball.
Then, roll it back and forth once or twice, to form it into an oval.
Lightly press the ends with two fingers and place on a lined baking sheet.
Finally, using a toothpick, press lengthwise into the center of each cookie, about halfway down, to get that coffee bean appearance.
Bake for 12 minutes, then let cool and enjoy!