Put the eggs, sugar and salt in a large heatproof bowl set over a pan of barely simmering water. Beat the mixture with a hand-held electric whisk for 2–3 minutes or until it triples in volume and thickens to the consistency of lightly whipped cream. Remove from the heat.
Sift in the flour and baking powder, add the lemon rind and juice and drizzle the butter down the sides of the bowl. Fold in gently, pour into a greased and lined 22 cm (8½ inch) square cake tin and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes or until risen, golden and coming away from the sides of the tin.
Meanwhile, put all the ingredients for the syrup in a small pan and heat gently until the sugar has dissolved. Increase the heat and boil rapidly for 4–5 minutes. Set aside to cool a little.
Remove the cake from the oven, leave it to rest for 5 minutes, then make holes over the surface with a skewer. Drizzle over two-thirds of the warm syrup. Leave the cake to cool and absorb the syrup.
Remove the cake from the tin and peel away the lining paper. Place the cake on a dish and serve in squares or slices with a heaped spoonful of crème fraîche or soured cream and an extra drizzle of syrup.