Preheat the oven to 350 degrees and line 2 cookie sheets with parchment paper.
Slowly melt the chocolate over medium-low heat in a saucepan, stirring every 30 seconds until melted and set it aside.
Whisk the flour, cocoa powder, baking powder, and salt in a large bowl.
Combine the butter and both sugars in a medium, microwaveable bowl and heat it for 90 seconds or until the butter is melted.
Stir the butter and sugar mixture well, and then stir in the espresso.
Add the eggs, egg yolk, and vanilla to the butter mixture and whisk until combined.
Pour the melted chocolate into the mixture and stir with a rubber spatula until well combined. Then, add the dry ingredients and mix again until just combined.
Use a 2 tablespoon cookie scoop to place the cookie dough about 2 inches apart on the cookie sheets.
Immediately bake the cookies for 9 to 12 minutes until the tops are crackly. You must bake them immediately to achieve this look.
Remove the cookies from the oven and let them rest for 30 seconds on the cookie sheet. Then, transfer them to a wire rack to cool completely.
Espresso Buttercream
Whip the butter in a stand mixer or hand mixer for 1 minute.
Add half the powdered sugar, salt, vanilla, and espresso and mix on medium for about 45 seconds or until blended.
Add the remaining powdered sugar and the heavy cream and mix on medium-high for 1 to 2 minutes until the frosting reaches your desired consistency.
If the frosting is too thick, add a small amount of heavy cream and mix it again. If the frosting is too thin, add a small amount of powdered sugar and mix it again.
Sandwich Cookies
Place half of the cookies top side down on a plate and pipe frosting on the top.
Top each frosted cookie with another cookie right side up.
Notes
StorageStore these cookies in a covered container at room temperature for up to 3 days. For longer storage, refrigerate them for up to a week in an air-tight container.Tips
Melt the chocolate chunks very slowly to avoid scorching them.
Be careful not to overcook your butter while melting it. You want to make it liquid without burning it.
Be sure to let the melted chocolate cool a bit before adding it to the other ingredients to avoid scrambling the eggs.
Sift the powdered sugar when making the buttercream frosting to ensure a rich, smooth texture.
Let the espresso cool before adding it to the frosting ingredients to avoid breaking the frosting.