In a large mixing bowl, whisk together the all-purpose flour, cornstarch, unsweetened black cocoa powder, baking soda, espresso powder, and salt until evenly combined.

In a separate bowl, cream the softened butter with granulated and light brown sugars for about 3-4 minutes, or until fluffy.

Mix in the room temperature eggs and vanilla extract, then gradually add the dry ingredients, stirring until just combined.

Chill the dough in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C) while the dough chills.

Line a baking sheet with parchment paper.

Using a cookie scoop, portion the chilled dough into uniform balls and place them on the prepared baking sheet, flattening them slightly for even baking.

Bake until the edges are set and the centers appear slightly soft, about 10 minutes.

Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

In a mixing bowl, beat the softened butter on medium speed for about 2 minutes until creamy.

Gradually add the confectioner’s sugar, along with the espresso powder, salt, vanilla extract, and Bailey's Irish Cream, beating until smooth and fluffy for about 3-5 minutes.

Once the cookies have cooled completely, generously spread the Bailey's buttercream on top of each cookie using a spatula or piping bag.

Dust half of the frosted cookies with unsweetened cocoa powder for a pleasing appearance.

Finally, place a coffee bean on top of each cookie for a decorative finish.