Pre-heat your oven to 200C/180C Fan/Gas Mark 6. Line an 8″ Square Pan with greaseproof paper.
In a small bowl, break up the white chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn’t burn.
In a large mixing bowl, whisk together the melted butter and both sugars. Add in the eggs and vanilla extract and mix again.
Whisk in the melted white chocolate until fully incorporated. Fold in flour and chopped chocolate. Pour into tin and spoon on the Nutella in random places. Swirl with spoon.
Bake 25-30 minutes (if it's still just a little wobbly, that's fine). Let cool. Optional: drizzle with warm Nutella/melted milk chocolate.