Preheat your oven to 175°C (350°F). Position a rack in the lower third of the oven. Do not grease your tube pan – the cake needs to climb the ungreased sides to reach its full height. You'll see the batter start to cling to the pan as it bakes, which is exactly what we want.

In a large, impeccably clean bowl, beat the egg whites on medium speed until they become frothy and look like a sea of tiny bubbles – this takes about 1 minute. Sprinkle in the cream of tartar and salt, then continue beating. The mixture will turn from translucent to white and start to hold soft peaks that flop over when you lift the whisk.

Gradually add half the caster sugar (100g), about a tablespoon at a time, with the mixer running on medium-high. You should hear the sound change from a sloshing rhythm to a smoother, thicker whir. Keep beating until the meringue holds stiff, glossy peaks – when you lift the whisk, the peak should stand straight up without drooping, and the surface looks shiny like satin.

In a separate small bowl, whisk together the remaining 100g caster sugar, the plain flour, and the cornflour. Sift this mixture directly over the meringue in three additions, folding gently with a large rubber spatula after each addition. You're aiming for a uniform colour – no white streaks of flour should remain. The batter will deflate slightly, but it should still look billowy and feel light, like a cloud you can scoop.

Stir in the vanilla extract, almond extract if using, and lemon zest. Fold just until you can smell the aromas – about 5 to 6 gentle strokes. Overmixing at this stage will make the batter dense, and you'll hear a slapping sound instead of a soft rustle as you fold.

Spoon the batter into an ungreased 25cm tube pan, spreading it evenly with the spatula. Run a thin knife through the batter to pop any large air pockets – you'll hear a faint popping sound as they burst. Bake for 32 to 38 minutes. The cake is done when the top springs back firmly when touched, the surface is a deep golden brown, and the kitchen smells like toasted vanilla and almonds.

Immediately invert the pan onto a wire cooling rack or bottle neck. Let the cake cool completely upside down – this prevents it from collapsing. You'll notice the cake pulling away from the sides of the pan as it cools, creating a small gap. This takes about 1.5 to 2 hours. Once fully cool, run a thin knife around the edges and the centre tube to release the cake.