1. Pre-heat the oven to 160°C. Line two flat baking sheets with greaseproof paper.
2. Place all the ingredients for the biscuits into a food processor fitted with a rotating blade and switch it on to high. Process until you have a smooth dough that leaves the bowl fairly clean.
3. Place the dough into a piping bag fitted with a large, star-tip piping nozzle and pipe an even number of rosettes or swirls onto the sheets. Try to make sure they’re all a similar size to make sandwiching them together easier later on.
4. Bake for 18-20 minutes or until lightly golden. Allow to cool on the sheet for 10 minutes, then transfer to a wire rack to cool completely.
5. To make the filling, beat the butter and sugar until pale and fluffy. Add the cocoa powder and chocolate and continue to beat for 10 minute.
6. Pile the buttercream into a piping bag fitted with a star tip and pipe onto the biscuits before sandwiching with another, or simply spread the buttercream on with a teaspoon.