Method

1. Preheat your oven to 180°C. Line a 20x20cm metal baking pan with parchment paper,
   leaving a 3cm overhang at each side.

2. Place chocolate and butter in a heatproof bowl and set it over a saucepan with just
   about ½ inch of simmering water. The bowl should fit nicely into the saucepan without
   touching the water. Stir frequently until melted and smooth. You can also melt
   it in the microwave at 50% power with frequent stirring to make sure it doesn't burn.

3. In a separate bowl, beat eggs with both sugars, salt and vanilla until pale, thick
   and fluffy using a whisk or an electric mixer. It will take a minute or two.
   Once it is lighter in colour (pale yellow), mix in the chocolate mixture until evenly incorporated.
   The batter should look thick and glossy at this point.

4. Sprinkle the flour over the batter and fold it through the chocolate batter (make sure
   you sift it if your flour and cocoa are lumpy). Fold it into the batter until evenly combined.

5. Scrape the mixture into your prepared pan and spread it out evenly.
   Bake for 27-30 minutes until puffed and shiny and a skewer inserted into
   the center comes out with a few moist crumbs attached. Transfer to a wire
   rack and let cool completely in the pan before slicing.