Chocolate Cookies

1. Preheat oven to 350°F (177°C). Meanwhile, line a large baking sheet with parchment paper or a silicone baking mat and set aside.
2. In a large bowl, add the unsalted butter, brown sugar and granulated sugar. Using a hand mixer or a stand mixer, beat on medium speed until creamy, about 2 minutes. Add in the egg yolk and vanilla extract. Beat again until combined. The mixture will now be light and fluffy.
3. In a separate bowl, combine the flour, cocoa powder, espresso powder and salt.
4. Add the dry ingredients into the wet ingredients about 4-5 tablespoons at a time and beat on low speed until combined. Repeat until all dry ingredients are added. The cookie dough will be slightly thick.
5. Beat the dough on low speed until it comes together and forms a smooth ball (see photo above). You can use your hands towards the end. However, make sure not to overmix as the butter will start melting.
6. Roll the cookie dough into 1-inch balls to form 12 cookies total. Place them on the lined baking tray or mat spaced 2-inches apart. Press your thumb gently into center of each cookie to form indentation, about 1/2-inch deep. 
7. Allow cookies to chill in the freezer, uncovered for 30 minutes or alternatively in the refrigerator, covered for 2 hours. This is very important to avoid your cookies from over-spreading while baking in the oven.  
8. Bake cookies at 350°F (177°C) for 10-12 minutes or until edges start to brown. Take them out of the oven, they will be soft and moist. To get perfectly round cookies, use a large round cookie or biscuit cutter to round out each cookie immediately after they come out of the oven.
9. Transfer cookies to a wire rack immediately and let them cool completely. These cookies will continue to bake while they cool.

Chocolate Buttercream

1. With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy – about 2 minutes.
2. Add confectioners’ sugar, cocoa powder, heavy cream, salt, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute.
3. Add 1/4 cup more confectioners’ sugar or cocoa powder if frosting is too thin or another Tablespoon of cream if frosting is too thick. Taste. Beat in another pinch of salt if desired.

Assembly

1. Pipe the buttercream onto the cookies as desired. I used a 1M star tip to make roses of chocolate buttercream on top of these cookies. 
2. Store these cookies in an airtight container in the refrigerator for upto 1 week.