There seems to be two schools of thought when it comes to brownies - some prefer them cake-like; others (like us) prefer them dense and rich. This version is studded with cacao nibs, hazelnuts and shards of bitter chocolate, with a hint of cinnamon and chilli to warm things up.
Note Cacao nibs are available from select health-food shops. Light muscovado sugar has a subtle molasses flavour. If it's unavailable, substitute brown sugar.