Heat the oven to 175°C (350°F). Line a 20 cm square pan with baking paper, leaving a little overhang on two sides so you can lift the brownies out later.

Put the chopped dark chocolate and butter in a heatproof bowl over a pan of gently simmering water, making sure the bowl does not touch the water. Stir now and then until melted and smooth, then take it off the heat.

Whisk the granulated sugar and brown sugar into the warm chocolate mixture until combined and glossy. Do not wait until the chocolate is cold — warm is what you want here.

Whisk in the eggs, one at a time, until each one is fully mixed in and the batter looks thick and glossy. Stir in the vanilla.

Sift in the flour, cocoa, and salt, and fold gently with a spatula until just combined — no white streaks, but do not overmix. Fold in the chocolate chips.

Pour the batter into the lined pan and smooth the top. Bake for 25 to 28 minutes, until the top is crackly and set, the edges are firm, and the centre still wobbles very slightly. Cool completely in the pan — at least 1 hour — then lift it out and cut into 16 squares with a hot, dry knife, wiping between cuts.