Let’s get cooking.
Heat the oven to 160°C fan or 180°C conventional (gas 4). Line the base and sides of a 20 cm square tin with baking paper, leaving an overhang so you can lift the brownies out later.
Chop the 200 g chocolate and cut the 200 g butter into pieces. Put both in a heatproof bowl over a pan of barely simmering water; the bowl must not touch the water. Stir occasionally until melted and smooth.
Lift the bowl off the pan and leave to cool until warm rather than hot, so the chocolate mixture will not cook the eggs when combined.
In a large bowl, whisk the 3 eggs and 175 g golden caster sugar until paler and thick enough that a trail from the whisk briefly rests on the surface. An electric whisk will make this easier.
Pour the cooled chocolate mixture into the eggs. Use a spatula to fold gently, lifting from the bottom and turning the mixture over, until the colour is even.
Sift in the 100 g flour, 30 g cocoa powder and salt. Fold just until no dry streaks remain, scraping the bottom of the bowl. Stop once combined to avoid overmixing.
Spread the batter evenly in the tin and bake on the middle shelf for 20–25 minutes. Start checking at 20 minutes: the edges should be set and a skewer inserted near the centre should have moist crumbs, not liquid batter. If still wet, bake for a few more minutes and check again.
Leave to cool completely in the tin; warm brownies are too soft to cut neatly. Lift out using the paper, transfer to a board and cut into 16 squares with a sharp knife, wiping it between cuts.
