First make the ganache, so that it has time to cool. Place the chopped chocolate into a bowl large enough to fit the chocolate and cream. Bring the cream to the boil in a saucepan, remove from the heat and allow the bubbling to stop, so that you don't risk scorching the chocolate.
Pour the cream onto the chocolate and stir until it's melted and evenly mixed.
Add the butter and stir until all of the butter has melted.
Cover the ganache with clingfilm, making sure it touches the surface, so that you don't get any condensation. Place in the fridge to cool. You don't want the ganache to set hard, just be firm enough to pipe or dollop.
Preheat the oven to 160C Fan/180C/350F/Gas 4, line two baking trays with greaseproof paper or use a silicone baking mat.
Put the butter into the bowl of a food mixer and beat until it's soft. Add the icing sugar and beat until light and fluffy. Gradually beat in the milk and then mix in the flour and cocoa powder.
Spoon the mixture into a piping bag fitted with a 10mm star piping tip. On the prepared baking tray, pipe the dough into 28 3.5cm circles. Bake in the preheated oven for 20 minutes.
Leave to cool and then sandwich two biscuits together with the hazelnut ganache, you can use the same piping tip as you used to pipe the biscuits, or you can just spread the ganache with a spoon.
Store in an airtight container, the biscuits will keep for about 5 days.