Preheat oven at 180c.Put all the biscuit ingredients into a food processor and pulse til mixture clumps together. Roll out the dough on sheet of baking parchment into a rectangular shape with a rolling pin. Cut the rectangle lengthways and across making smaller rectangles if you like use the end of skewer to mark the biscuits with dots like the traditional bourbon. Chill it for 30 minutes then bake it in oven for 25 minutes,then carefully cut through the marks to separate the biscuits let them cool. To make filling sift icing sugar and cocoa in a bowl with the butter mix until well combined. Finally sandwich the cool biscuits with the filling.