Put the egg white and cream of tartar in the bowl of an electric stand
mixer. Using the whisk attachment, whip on speed 8 (under high)
until the whites have changed from foamy to stiff and white. Just after
you have started the whites, put the buttermilk, oil, egg yolks and
finely grated lemon zest in a wide b owl and combine with a whisk.

Start adding the reserved sugar to the whipped whites in four
additions. It is imperative to the success of the cake that the meringue
forms very stiff–medium peaks. Reduce to speed 1 (low) for 1 minute
to even out the air bubbles.

Sift the dry ingredients over the buttermilk/yolk/oil mix and whisk
in by hand to form a loose batter, then gently and thoroughly fold
in one-third of the meringue. Gently fold in the remaining meringue
until no white streaks remain. Pour the mix into the ungreased angel
food cake tin. The batter should come up to about 5 cm (2 in) from
the top of the tin.

If the batter sits higher, it may bake and soufflé a little over the sides. To
avoid this, give the batter a swizzle with a small knife in the tin.

Wipe any batter smears that are around the side of the tin or on the
centre tube, as they can impede the rise.

Bake for 55–60 minutes until golden and puffed. The cake around
the inner tube should have a dry appearance. Remove from the oven
and immediately invert the centre tube onto a bottle – carefully and
with courage! Let it hang for 2–3 hours, until the base of the tin is
completely cool to the touch.*

While the cake bakes, start the citrus crisps so you can get them in
the oven when the chiffon comes out.

Make the frosting. Strain and weigh the lemon juice, then reduce (see
Reduce the juice, page 21) to a quarter of its original volume and stir
into the softened Fluffy yoghurt frosting before topping the cake.

To release the cake, run a thin knife or metal offset spatula around
the edge of the tin. This may take a little practice. Start where one of
the legs is soldered on and shimmy the knife down alongside the rivet
(if it’s easier, lay the tin on its side to do this). Make sure the knife  is
super close to the tin and then, with a slight angle, drag the knife
around the side of the tin to the next leg point. Repeat this action
at each leg point. Do not run a knife around the centre tube – it will
naturally release when cut from the base.** Turn the chiffon cake out
onto a serving plate and cut it from the base plate.