Preheat the oven to 180C / 350F / 160C fan forced. Line 2 baking trays with baking paper.
Add the flour, baking powder, cocoa and sugar to processor and process to combine. Add the cold butter and process again for another 20 – 30 seconds until it is finely chopped through the flour mixture. It will still be a little lumpy and that’s ok. Finally, add the egg, vanilla and half of the milk.
Process again for about 30-40 seconds until everything well combine and the dough starts forming large clumps. If it’s not quite sticking together, add the rest of the milk and blitz again.
Tip the mix out onto a clean surface and pull together with your hands, kneading lightly until it starts to stay together – only a minute or so.
Lay out a long sheet of baking paper (about 70cm) and lay the dough in the centre of one half of it. Fold the other half over the top and use a rolling pin to roll the dough out to about 4-5mm thick.
Sit in the fridge for ½ an hour before proceeding.
Use a 2 ½ inch circle cutter to cut as many cookies as you can. Carefully transfer them to a baking tray about 1 inch apart. Re-roll the dough and repeat until it has all been used.
Bake in the oven for 6 minutes, turn the trays, bake for another 5-6. Transfer them to a wire rack and allow to cool completely.
Now dump the filling ingredients (except the extra cocoa), into the food processor, sugar first. Process the whole lot for about 3 minutes on the low setting. If it’s too firm to pipe add a little more cream. Too loose to pipe, add a little more icing sugar. Transfer the buttercream to a piping bag.
Either pipe or spread the buttercream over half of the cookies. Place the unfrosted halves on top and gently press down. Sift the extra cocoa over the top.