Grind the pistachio nuts in a food processor. Grease a 20 cm (8 inch) sandwich tin or springform tin. Line the bottom with greaseproof paper.
Cream together the butter and sugar until smooth and creamy. Beat in the ground pistachio nuts, flour, cocoa powder, baking powder, eggs and milk. Mix until evenly combined.
Pour the mixture into the prepared cake tin and spread in an even layer. Scatter the chocolate pieces and extra chopped pistachio nuts over the top and place on the lower rack of the halogen oven.
Set the temperature to 175°C (347°F) and cook for about 25 minutes until the top feels firm to the touch. (A skewer inserted into the centre should come out slightly moist.) Leave to cool in the tin for 10 minutes before transferring to a plate. Dust with extra cocoa powder and serve warm with lightly whipped cream.