Preheat the oven to 170c/150c (fan)/gas 3. Butter a 23cm loose-bottomed cake tin and line with parchment paper.
Melt the butter and chocolate with the salt in a heatproof bowl set over a pan of barely simmering water. Stir occasionally but not vigorously. Once the butter and chocolate has melted, remove the pan from the heat but keep the mixture warm and resting over the pan of water until ready to use.
Separate the eggs, placing the yolks into the bowl of your electric mixer first. Add half of the caster sugar and whisk until the mixture forms pale and fluffy ribbons and has doubled in volume. Remove the bowl of melted chocolate from the pan of hot water and set it on your worktop. Fold the whisked yolks into the melted chocolate. The should be marbly and not fully incorporated.
Wash out the mixer bowl and dry it thoroughly, and now add the egg whites to it along with the remaining sugar. Whisk on a high speed until medium-soft peaks form. Do not overwhip. The consistency of the egg whites should resemble that of the yolk and chocolate mixture. Fold the whites into the chocolate until just mixed, then pour into your prepared pan.
Bake for 30 to 40 minutes, until the top of the cake is puffed and just starting to crack. The cake will still have a bit of a wobble and will be puffing out over the top of the pan. Place the pan on a wire cooling rack and coax any extra overflowing cake back into the pan. Allow to cool for a good 20 minutes, then remove the sides of the pan, peel off the paper, and slide the cake onto a nice serving plate. Dust with cocoa powder before serving.
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