1. Add flour, confectioners’ sugar, cocoa powder, and salt to a large bowl. Add the butter in small pieces and mix to combine. Rub everything between your fingers and knead until you get a smooth dough. Shape the dough into a log with a diameter of 2 inches (5cm), wrap into plastic wrap, and place in the fridge for at least 2 hours (or overnight).

2. Preheat the oven to 285°F (140°C). Line a baking sheet with baking parchment and set aside. Take the dough out of the fridge and cut it into thin slices (about 28). Place the dough slices with some space in between on the baking sheet – place the remaining dough that did not fit on the baking sheet back in the fridge. Bake the cookies for 22-24 minutes. Take out of the oven and let cool down on the baking sheet for some time, then transfer to a wire rack and let cool down completely. Repeat the process with the remaining dough.

3. For the filling add peanut butter, butter, confectioners’ sugar, and salt to a large bowl and mix for 3-4 minutes (with a handheld mixer) until well combined and smooth. Add some heavy cream and mix in if the mixture seems too thick. Add a small amount of buttercream on a cooled cookie and sandwich it with a second cookie. Repeat until all buttercream and cookies are combined. Store the sandwich cookies in a cool place.