Flourless Fudgy Brownies

Heat oven, line tin. Heat the oven to 175°C (155°C fan), allowing at least 20 minutes for it to come fully up to heat before baking. Line the 20cm metal tin with a baking-paper sling; a metal tin bakes more evenly than glass, which tends to leave these delicate brownies drier at the edges.

Melt chocolate and butter. Melt the chocolate and butter together in a heatproof bowl set over a pan of barely simmering water, stirring now and then until smooth. Take it off the heat and leave to cool for about 5 minutes so it won't cook the eggs later.

Whip eggs and sugar. Whisk the eggs, sugar, salt and vanilla on medium-high speed for 5-6 minutes, until pale, thick and roughly doubled in volume, leaving a ribbon trail when you lift the whisk. This whipped foam does the job flour would normally do, trapping air that sets into structure and giving the shiny, crackly top.

Combine and fold. With the mixer on low, pour in the warm chocolate mixture in a slow stream, then fold in the sifted cocoa powder until no streaks remain. Folding gently keeps as much of that whipped air in the batter as possible.

Bake. Scrape the batter into the tin, level it, and bake for 25-30 minutes. The top should look set and crackly with a centre that barely wobbles; a skewer should come out with moist crumbs rather than wet batter, and a probe reads around 90-96°C in the middle.

Cool and chill. Cool completely in the tin, then chill for 1 hour before cutting. Flourless brownies have very little structure to hold them together while warm, so chilling firms them up enough to slice cleanly.

Cut into squares. Cut into 16 pieces using a hot knife, wiping the blade between cuts for neat edges.