Preheat the oven to 180C/350F/gas mark 4. Lightly grease two 7-inch Cake tins and line with greased baking parchment.
Separate the eggs into two large bowls. Add the sugar to the yolks and whisk (preferably with an electric whisk) until the mixture has doubled in size and is pale and fluffy. Add the lemon zest and juice (or the vanilla essence, if using).
Wash and dry the whisk then whisk the egg whites to stiff peaks. Using a large metal spoon, begin folding a spoonful of the whites into the yolk mixture, alternating with the sifted flour and baking powder, and ending with the whites. Do this gently so as not to lose valuable air.
Divide the mixture between the two tins and bake for 20?25 minutes. When done, they should be golden on top, springy in the centre and come away from the sides.
Cool in the tins for a couple of minutes. Lay a sheet of greaseproof paper on a work surface and sprinkle with sugar. Turn the cakes upside down on to this and transfer to a wire rack to finish cooling. Sandwich together with jam or lemon icing and decorate the top accordingly. Eat as fresh as possible.