Heat the oven to 180°C/Gas Mark 4. Line a 20cm square cake tin with non-stick parchment. Melt the butter and Chocolate over a very low heat in a saucepan, then leave to cool for 10 mins. Stir in the beaten eggs.
Sift the flour, Cocoa Powder, Baking Powder and sugar into a bowl, add the chocolate chunks and pour into the butter mixture and mix well.
To make the cheesecake, mix all the ingredients together in a large bowl until well combined.
Pour the brownie mix into the prepared tin a blob the cheesecake into the brownie mix, use a cocktail stick to swirl the cheesecake through the brownie. Bake in the oven for 25-30 minutes, until the brownie is baked – the centre of the brownie should be just set and not wobble when the brownie is baked. Remove from the oven and leave to cool.
Once cooled, melt the Extra Dark Chocolate in the microwave. Cut the brownie into 12 squares and drizzle with the melted chocolate.