Preheat the oven to 180C Fan/355F Fan (or 200C/400F if not fan). Generously butter a 23cm (9-inch) round springform tin with butter and line the base with non-stick baking paper. Set aside.
Melt the butter and chocolate together in a bowl. I usually do this in the microwave in 30 second bursts, stirring well each time until smooth. Or you can place in a heatproof bowl over a pan of "just" simmering water, ensuring the bottom of the bowl doesn’t come into contact with the water (or the chocolate will burn).
Once you've got a smooth, glossy mixture, pop a sieve over the bowl and sift in the cocoa powder and salt. Stir until smooth. Set aside.
Separate the eggs, putting the whites into the bowl of a stand mixer and the yolks into another large bowl.
Add 135 g caster sugar to the egg yolks and whisk using an electric whisk for about 2 minutes - or until pale, light and thick.
Meanwhile, using the stand mixer (or you can use the electric whisk - it just needs to be really clean otherwise the egg whites won't whisk in the way you want them to), whisk the egg whites until you have soft peaks. Gradually add the remaining sugar, whisking until the egg whites are stiff and glossy.
You should now have three bowls that need to be mixed together. First, pour the melted chocolate mixture into the egg yolks and stir gently. Then add two spoons of the whisked egg whites into the chocolate mixture and stir quite vigorously so it is well incorporated. This is often called the ‘sacrificial spoon’ as we don’t worry too much about keeping in the air at this stage.
Next, gently fold in the remaining egg whites, taking care to keep in as much air as possible. You shouldn’t see any of the egg whites after you’ve folded it together.
Pour ⅔ of the cake batter into the prepared tin and bake for about 25 minutes - or until the cake is well risen, puffy and a skewer inserted into the centre comes out clean. Leave to cool completely in the tin.
Meanwhile, pour the remaining ⅓ of the cake batter into a medium bowl and set aside, or cover with cling film and pop into the fridge if using much later. This will become the molten chocolate top layer of the cake.
Preheat the oven, once again, to 180C Fan and line a baking tray with non-stick baking paper. Set aside.
Once the cake has cooled completely, remove from the tin and place on the prepared baking tray. Add the reserved ⅓ of the cake batter on top of the cake - you can pour it or, if it has been in the fridge, gently spread using a palette knife.
Bake the cake for about 10-15 minutes or until you see a thin crust on the top but the middle is still soft and jiggly underneath. Remove from the oven and leave to stand for 10 minutes.
Cut into slices and serve warm with double cream, ice cream or custard. You can also serve it chilled from the fridge.
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