Preheat the oven to gas mark 6/200°C/180°C Fan/400ºF. Line and butter two 21cm / 8 inch sandwich tins.Mix the egg yolks, 100g / ½ cup 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 don't even have enough to cover the bottom of the tins, but don't 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 200g / 1 cup of 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 1 teaspoon sugar over the peaks. Put the tins into the oven for 20-25 minutes.With a cake-tester, pierce the cake that has the flat meringue topping to check it's cooked all 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 surface of the first, waiting, cake with the lemon curd and than spatula over the cream and top with the remaining cake, bronze-peaked meringue uppermost.