Preheat oven to 180°C/350°F (160°C fan forced).
Spray a 20cm/8" square tin with oil and line with baking/parchment paper with overhang
Melt chocolate and butter over a double boiler
Add sugar, mix, then add eggs and mix well until smooth and molten.
Add flour, pinch of salt & stir until smooth.
Stir in chopped chocolate if using, pour into pan.
Bake 22 minutes for really gooey in the center, 25 minutes for fudgy but still very moist or 32 minutes for moist fudge-cake-like.
If you didn't use the extra chocolate for stirring in, reduce cook time by 2 minutes.
Rest for 10 minutes before lifting out of the pan. Allow to cool for at least 20 minutes before cutting.