Preheat the oven to 350oF (180oC). Butter the sides and bottom of a 9x13-inch and line the pan with parchment paper.

In a medium bowl, sift the flour, salt, and cocoa powder together and set aside.

Put the chocolate and butter in a large bowl and melt it gently in the microwave or set it over a saucepan of simmering water, stirring occasionally, until the chocolate and butter are completely melted and smooth. Remove from the heat and set aside to cool a little.

Add the sugars to the melted chocolate. Whisk until completely combined. The mixture should be room temperature.

Whisk the eggs and vanilla in a jug together. Add a little of the eggs to the chocolate mixture and whisk until combined. Add the remaining eggs and whisk until combined. Do not overbeat the batter at this stage or your brownies will be cakey.

Sprinkle the flour mixture over the chocolate mixture. Using a rubber spatula (not a whisk), fold the flour mixture into the chocolate until just a bit of the flour mixture is visible.

Pour the batter into the prepared pan and smooth the top. Bake in the center of the oven for 30 minutes (just 30, no more no less).

Let the brownies cool completely, then lift them out of the pan using the parchment paper.

Store at room temperature in an airtight container.