Preheat the oven to 180 deg C and line a 3x3 inch square tin or an 8 inch loaf tin and set aside.
In a bowl add the butter, dark chocolate and melt in the microwave for 20 second intervals, stirring between each interval - until smooth. Allow to cool.
In a large mixing bowl add the egg, sugars and vanilla, whisk until pale, creamy and smooth. Around 2-3minutes, you can do this with an electric beater as well.
Add the melted chocolate and butter mixture, whisk until combined.
Add the flour, corn flour, cocoa powder, salt and fold together until combined.
Pour into the prepared tin and bake for 12-15 minutes until the top is shiny, crinkly and a tooth pick inserted into the center comes out with damp/moist/sticky crumbs.
Allow it to cool before cutting into 4 equal pieces.