Prep your tools. Preheat the oven to 325°F. Line two cookie trays with parchment paper and set aside.

Combine the dry ingredients. In a medium bowl whisk together: flour, cocoa powder, baking soda and salt.

Combine the wet ingredients. Fit a stand mixer with a paddle attachment. Cream the butter with the sugars until light and fluffy (approx. 5-7 minutes on medium-high speed). Scrape down sides of the bowl 3 times in the process.

Add the egg, vanilla, buttermilk and mix for 30 seconds, scrape down sides of the bowl and mix just until combined. Do not over mix.

Mix the dry ingredients into the wet ingredients. Add the flour mixture in two batches, mixing until incorporated and scraping down the bowl in between the additions. The dough should be thick and dense.

Portion out the cookie dough. Scoop out ½ tbsp (0.5 oz) of the dough, shape into a bowl and drop on the cookie sheet.

Repeat with the remaining dough, dropping it a few inches apart. You should get 38 cookies, which will make 19 cookie sandwiches.

Bake. Bake at 325°F for 7 minutes. The cookies should still be slightly soft to the touch in the middle at the end of the baking time. Leave them to rest and cool completely on the baking trays.

Make the filling. To make the filling, cream the butter with icing sugar on medium speed in a bowl of a stand mixer fitted with the paddle attachment or an electric hand mixer. This should take approx. 5 minutes. Add vanilla extract and mix for a few seconds just until combined. The filling should be smooth and creamy.

Fill the cookies. Place the filling in a piping bag, and pipe out a portion onto the center of each of the 19 cookies. Cover with the top cookie and gently press down. Place the cookies in an airtight container and refrigerate for an hour before serving, or serve slightly soft!