Melt the butter gently and leave to cool slightly. Use a little of it to grease 2 round 18cm cake tins. Dust the sides generously with flour, tip out any excess, then line the bases with baking parchment.
Sift the flour and salt together; set aside. Put the eggs and sugar in a large heatproof bowl that will sit snugly over the top of a saucepan. Pour boiling water into the saucepan and sit the bowl on top. The hot water must not actually touch the bowl; it is the steam that is needed to heat it up, which helps the sugar to dissolve and thickens the mixture slightly.
Using a hand-held electric beater, whisk the eggs and sugar for about 10 minutes, until very pale, thick, moussey and at least trebled in bulk. The mixture should hold its shape on the surface for a few seconds if you let a little fall from the beaters.
Sift half the flour into the egg and sugar mixture and gently fold it in with a large metal spoon. Repeat with the remaining flour. Carefully pour in the melted butter and fold this in too, until just incorporated. Pour the mixture into the tins and bake in an oven preheated to 180°C/Gas Mark 4 for 25 minutes, until golden brown and firm to the touch. Leave to cool in the tins for a few minutes and then, running a small, sharp knife around the edge of the cakes if necessary to help release them, turn them out on to a wire rack to cool completely.
Serve topped with, or sandwiched together with, lashings of lightly whipped double cream and fresh raspberries or strawberries. Finish with a sprinkling of vanilla sugar or plain sugar.
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