Ptichye Moloko

Heat the oven to 200C/400F.

Butter a 22cm springform tin and line the base with baking paper.

Whisk the 2 whole eggs with the 100g of caster sugar for 6-8 minutes, until the mixture has tripled, gone very pale and holds a ribbon on the surface for about three seconds.

Sift the flour over in two additions and fold it in with a spatula, cutting down through the middle and lifting from the side.

Fold in the 20g of melted butter last, pouring it down the inside of the bowl rather than into the centre so it does not knock the air out.

Spread the batter into the tin and bake for 12-15 minutes, until it springs back under a fingertip and the edges have shrunk from the sides.

Cool it completely, then level the top with a serrated knife so you have a flat layer about 1cm thick.

Wash and dry the tin, set the sponge back in the base and line the sides with a collar of baking paper standing 8cm proud.

Stir the agar powder into the 300ml of water in a heavy pan and leave it for 20-30 minutes to hydrate.

Add the 460g of caster sugar to the agar and set the pan over medium heat, stirring until the sugar dissolves.

Bring it to a full rolling boil at 100C/212F and hold it there for at least a minute.

This is the step that decides whether the cake works at all. Agar does not activate until it has actually boiled, which is the exact opposite of gelatine, and a syrup that was merely brought hot will never set no matter how long it is chilled.

Boil for a further 5-8 minutes, until the syrup reaches 110C/230F and falls from the spoon in a thick unbroken thread.

While the syrup boils, whisk the egg whites with the lemon juice and the fine salt to firm glossy peaks.

Lemon juice rather than cream of tartar is the usual acid here, and it steadies the foam against the shock of boiling syrup.

Beat the 200g of softened butter with the condensed milk and vanilla until pale and completely smooth, and leave it at room temperature.

The butter has to be genuinely soft. Cold butter seizes into hard flecks the moment it meets the meringue and there is no way of beating them back out.

With the mixer running at medium speed, pour the boiling syrup into the whites in a thin steady stream down the inside of the bowl.

Keep it off the whisk. Syrup poured onto the moving whisk is flung against the cold sides of the bowl, where it sets instantly into agar threads that stay in the finished cake.

Whip for 4-5 minutes more, until the meringue is thick, glossy and has cooled to about 60C/140F, which is roughly the point where the bowl stops being too hot to hold.

Add the butter mixture in three additions on low speed, beating each one only until it disappears.

Work quickly from here. Agar sets at around 40C/105F, far warmer than gelatine, and the mixture stiffens in the bowl within a few minutes of the butter going in.

Pour it straight onto the sponge and level the top with an offset palette knife in as few strokes as you can.

Leave it at room temperature for 30 minutes, until the surface is set and dry to a light touch, then refrigerate for 2 hours.

Melt the chopped chocolate with the 80g of glaze butter in a bowl over a pan of barely simmering water, stirring until smooth.

Let the glaze cool to about 30C/86F, at which point it is barely warm on the lip.

Pour it over the cold set soufflé and tilt the tin to run it into an even sheet.

Do not pour it warmer than that. Hot glaze melts a channel into the surface and sinks through in patches instead of sitting on top.

Refrigerate for 30 minutes, until the chocolate is firm.

Release the ring and peel the paper collar away from the sides.

Cut with a knife dipped in hot water and wiped dry between every slice, otherwise the chocolate shatters and drags.

Serve cold, and keep it refrigerated for up to four days.