Weigh and arrange all ingredients for use.
In a pan, melt the dark chocolate with the butter over a low heat, stirring.
Mix the sugar, salt and vanilla sugar in a bowl and stir in the melted chocolate.
Gradually beat in the eggs.
Mix everything until smooth.
Sieve in the flour and cornflour and fold into the chocolate mixture.
Preheat the oven to 180°C (160°C fan, gas 4). Line a cake tin with baking paper. Spoon the chocolate mixture into the tin and spread level.
Scatter over the chocolate chips.
Bake in the oven for about 25 minutes. The cake should still be slightly crusty on top but still slightly moist inside. Allow to cool completely.
Turn out the brownie and cut into pieces.