Prepare the Cookies: Cream together the butter and sugar until pale and fluffy. Scrape down the bowl, add the egg and continue beating until the egg is incorporated and the mixture lightens in color. Add the cocoa powder and beat until there are no lumps. Finally, add the flour and salt on low speed until incorporated. The dough will be very soft. Turn the dough onto a piece of parchment paper on a work surface, top with another piece of parchment paper, and roll out into a large rectangle about ¼-inch thick. Put the dough on a tray and place in the freezer for 30 minutes.
Preheat the oven to 350°F (180°C) with a rack in the center position and line two sheet pans with parchment paper.
Take the dough out of the freezer and slice it into 28 small (1¼ x 2½ inch) rectangles. Seperate the rectangles and spread them out on the lined baking trays. Bake for 10 minutes, then transfer to a wire rack to cool completely.
Make the Filling: Cream the butter until soft, sift in the remaining ingredients and beat until well combined. Spread a heaped teaspoon of filling onto half of the cookies, on the bottom side. Top with the remaining cookies, then place in the refrigerator to chill while you prepare the coating.
Prepare the Coating: Place the chocolate and coconut oil in a heat-proof bowl and melt together in the microwave on High (100%) in 30-second bursts, stirring well after each burst. Once the chocolate is mostly melted, remove and stir gently until smooth.
Take a chilled cookie and carefully coat it in the chocolate – this is easiest if you use your hands – then put it on a lined baking tray. Chill in the refrigerator completely before enjoying.
Reprinted with permission from The Sugar Hit!, Hardie Grant 2015.