Preheat the oven to 180°C (350°F) and line a 9"x9" baking pan with baking paper.
Melt the butter, milk chocolate, dark chocolate and Nutella together.
Place the eggs, vanilla extract, brown sugar and sugar in a bowl and beat together until pale.
Pour in the chocolate mixture into the egg mixture and mix together.
Add in the cornflour and cocoa and mix until all combined.
Pour into the prepared baking pan and dollop of teaspoon sized dollops of the remaining Nutella throughout. Place in the oven for around 35-40 minutes or until a skewer inserted comes out with loose crumbs attached.