Preheat the oven to 200 °C.
Line and butter two 21 cm sandwich tins.
Mix the egg yolk, 100 g of the sugar, the butter, flour, cornflour, baking powder, bicarb and lemon zest in a processor.
Add the lemon juice and milk and process again.
Divide the mixture between the prepared tins. You will think you do not even have enough to cover the bottom of the tins, but do not panic. Spread calmly with a rubber spatula until smooth.
Whisk the egg whites and cream of tartar until peaks form and then slowly whisk in 200 g sugar.
Divide the whisked whites between the two sponge-filled tins, pouring, or, more accurately, spreading the meringue straight on top of the cake batter.
Smooth one flat with a metal spatula, and with the back of a spoon, peak the other and sprinkle a teaspoonful of sugar over the peaks.
Put the tins into the oven for 20 to 25 minutes.
With a cake-tester, pierce the cake that has the flat meringue topping to check that it is cooked right through. (It will have risen now but will fall back flattish later.) No sponge mixture should stick to the tester.
Remove both cakes to a wire rack and let cool completely in the tins.
Unmould the flat-topped one on to a cake stand or plate, meringue side down.
Whisk the double cream until thick but not stiff and set aside.
Spread the flat sponge face of the first, waiting, cake with the lemon curd and then spatula over the cream and top with the remaining cake, bronze-peaked meringue uppermost.
NOTE: After I had made this a couple of times, it occurred to me that if I were to fill the cakes with passon fruit curd in place of the lemon curd which I buy, but it must be a good-quality one), it would be particularly appropriate for Easter.