Preheat the oven to 180°C/gas mark 4. Line 2 baking trays with baking parchment.
Place the dark chocolate in a heatproof bowl over a pan of simmering water (make sure it's not touching) until melted. Leave to cool slightly.
Using an electric mixer, whisk together the butter, light brown sugar, caster sugar and vanilla extract until creamy. Whisk in the melted dark chocolate and egg.
Stir in the flour, cocoa powder and baking powder, until well combined. Stir in the chocolate chunks. Spoon rounds of the mixture on to the trays and bake for 12 minutes.