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 (Note 2).
Place butter and chocolate chips in a ban marie
Add sugar and vanilla, mix, then add eggs and mix well until smooth and molten.
Add flour, cocoa and salt and stir until smooth. Stir in chopped chocolate or other ingredients, pour into pan.
Bake 24 minutes for really gooey in the centre, 28 minutes for fudgey but still very moist (my favourie, shown in video & photos), 32 minutes for moist fudge-cake-like. We always do 24 min. If not using extra ingredients, remove 2 min.
Rest for 10 minutes before lifting out of the pan. Allow to cool for at least 20 minutes before cutting. Store in an airtight container for 4 days (bet they don't last that long!) or freeze for 3 months.