Weigh the flour and salt into a small bowl and set aside with a sieve
on top.

Put the egg white and cream of tartar in the bowl of an electric stand
mixer. Using the whisk attachment, whip on speed 6 (medium–high)
for 1–2 minutes, until the whites have changed from foamy to stiff
and white.

Start adding the sugar to the whites very gradually – about 1 heaped
teaspoon every 20 seconds, so the whole process takes 3–4 minutes.

It is imperative to the success of the cake that the meringue is so stiff
you could carve a knife through it. After all the sugar is in, reduce to
speed 1 (low) for 1 minute to even out the air bubbles.

While the meringue does the slow whip, sift the dry ingredients over
the butter/milk/yolk mix and whisk in by hand to make a smooth paste,
then gently and thoroughly fold in one-third of the meringue. The
super-stiff meringue will be hard to blend at first – just keep folding,
shaking the batter off the whisk from time to time. Gently fold in the
remaining meringue and do a final fold with a plastic spatula until no
white streaks remain. Scrape the mixture into the tin and smooth the
top, making a recessed rectangle with a 2 cm (¾ in) border – think
photo frame.

The recess controls doming so you achieve an even and flat sponge.

Bake for 15–18 minutes until the top is pale gold. While the cake
bakes, set up a cooling rack on your work surface and spray it with
cooking oil. As soon as the cake is done, quickly run a knife along
the unpapered sides and confidently flip the cake from the tin onto
the cooling rack. If you flip slowly, the cake can get squished or
tear.* Gently peel away the paper and set the rack over your sink (or
carefully elevate) to get maximum cooling airflow beneath.

While the cake cools, start the White chocolate and mascarpone
mousse so it is still pourable when you reach assembly time. If the
mousse sets, you can still assemble but the sides won’t be as smooth.
continued …
Filling
1 × batch White chocolate
and mascarpone mousse
(page 257) – start this after
the sponge is baked so the
gelatine is still flexible
160 g (5½ oz) strawberry jam
(homemade or very good
store-bought)
750 g (1 lb 11 oz) fresh,
perfectly ripe strawberries
Strawberry sherbet dust
20 g (¾ oz) icing
(confectioners’) sugar
5 g (⅛ oz) freeze-dried
strawberries
2 g (¹⁄₁₆ oz/¼ teaspoon) citric
acid