Heat the oven to 180 C / 355 F. Butter a 20 cm round cake pan and line the base and sides with parchment paper, then butter the parchment.

Set a heatproof bowl over a saucepan of barely simmering water (bain-marie). Add the chopped chocolate and butter cubes. Stir occasionally until fully melted and smooth, about 5 minutes. Remove from heat and let cool to about 40 C / 104 F (warm to the touch but not hot).

Whisk the 4 egg yolks with 100 g of the caster sugar in a medium bowl until the mixture turns pale and slightly thickened, about 2 minutes.

Pour the cooled chocolate mixture into the yolk mixture and stir until fully combined. Sift in the flour and fold until no dry streaks remain.

In a separate clean, grease-free bowl, beat the 4 egg whites with an electric hand mixer until foamy. Gradually add the remaining 50 g caster sugar and continue beating until soft peaks form - the whites should hold a gentle curve when you lift the beater, not stand stiff.

Add one third of the beaten whites to the chocolate batter and stir to loosen the mixture. Gently fold in the remaining whites in two additions, using a rubber spatula, until just combined with no white streaks visible.

Pour the batter into the prepared pan and smooth the surface gently. Bake for 22 to 25 minutes, until the top looks dry and slightly cracked but the center still has a noticeable wobble when you shake the pan.

Remove from the oven and let the cake cool completely in the pan, at least 1 hour. The center will continue to set as it cools. Once cool, run a knife around the edge, remove from the pan, and dust lightly with powdered sugar just before serving.