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 smooth and no lumps appear. Add the flour and salt until fully incorporated. The dough will be very soft. Turn the dough onto a piece of parchment paper or wax paper. Top with another piece of parchment paper, and roll the dough out into a large rectangle about 1/4-inch thick. Put the dough on a baking tray and place in the freezer for 30 minutes.
Preheat the oven to 350 degrees F. (180 degrees Celsius) and line two baking sheets with parchment paper.
Take the dough out of the freezer and slice it into 28 small 3x6 cm (1 1/4 x 2 1/2 inch) rectangles. Separate the rectangles and spread them out on the prepared baking trays. Bake for 10 minutes, then transfer to a wire rack to cool completely.
To make the filling, cream the butter until soft, add in the remaining ingredients and beat until well combined. Spread a heaping teaspoon of filling onto half of the cookies. Top with the remaining cookies, then place in the refrigerator to chill while you melt the chocolate for the coating.
For 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. You may also melt the chocolate on the stove over a double broiler.
Take a chilled cookie and carefully coat it in the chocolate - I like to use a fork and then allow any excess chocolate to drip off back into the bowl. If desired, top each bar with chopped peanuts and or toasted pumpkin seeds.
Chill in the refrigerator and then dip one more time through the melted chocolate if desired. Place back on the baking trays and chill until the chocolate is solid...then try and stop yourself from eating them all immediately!
*Reprinted with permission from , Hardie Grant 2015