While the chocolate melts, start the egg yolk foam. Whip the egg
yolks, dark brown sugar and salt in the bowl of an electric stand mixer
with the whisk attachment on speed 8 (under high) for 5 minutes
until pale brown and thick. If you are a one-bowl house (like me),
scrape the foam out into a small mixing bowl and set aside.

Egg yolk and sugar foams are sturdy and can hold more than egg white–
based ones.

Weigh the cocoa into a small bowl and set aside with a sieve for later.

Weigh the crème fraîche into a container to heat it in (stovetop or
microwave) and set aside for later.

Clean the stand mixer bowl well and put the egg whites and cream of
tartar in. Using the whisk attachment, whip on speed 8 (under high)
until the whites have changed from foamy to stiff and white – around
2–3 minutes.

Start adding the caster sugar to the whites – VERY gradually – about
a heaped tablespoon every 30 seconds, so the whole process takes
around 3–4 minutes. It is imperative to the success of the cake that
the meringue is very stiff and shiny. Reduce to speed 1 (low) for
1 minute at the end to even out the air bubbles.

While the final sugar is being added to the meringue, return to building
the chocolate base. Check your chocolate/butter mix is quite warm,
then fold in the egg yolk foam. Sift over the cocoa powder and fold in.

Heat the crème fraîche until melty/steamy (it doesn’t matter if it looks
curdled) and fold that in too. Finally, fold in one-third of the whipped
meringue. When the first addition is almost completely incorporated
(still streaky), fold in the remaining meringue until very few streaks
remain. Do a final fold with a clean flexible plastic spatula to eradicate
any final streaks. The mix should look fluffy, like chocolate mousse.

Scrape the batter carefully into the lined tin. Smooth the top with an
offset spatula.

Adding the second portion of whites/meringue before it’s completely
incorporated maintains more air overall. Concentrate on making the batter
streak-free after the final whites addition.

Bake for 45–55 minutes. The cake is cooked when there are puffed
cracks at the edges, the top is matt dark brown and a poke of the
tin produces a firm-set wobble (internal temperature 80–85°C/
176–185°F).

If you are using a skewer to test the cake, there will be batter on the skewer
but it will look gooey, not liquid.