Preheat the oven to 180C/355F/Gas 4.
Place the eggs and sugar into a bowl and beat until pale and creamy.
Add the melted butter and stir well.
Sift the flour and cocoa into the egg mixture, then add the melted chocolate and the hazelnuts and stir well.
Pour the mixture into a greased 20cm/8in ovenproof dish and transfer to the oven to bake for 30-35 minutes, or until the surface is set but a skewer comes out with a little of the mixture clinging to it. Remove from the oven and allow to cool.
For the marshmallow sauce, place the cream, sugar and vanilla seeds into a saucepan over a low heat and slowly bring to the boil. Simmer for about five minutes to thicken the liquid, then remove from the heat.
Add the marshmallows to the cream mixture and stir well to melt.
To serve, cut out portions of brownie and place onto plates. Pour the sauce over.