Sift the flour into a bowl. Add the butter and rub into the flour until well blended and the mixture resembles fresh breadcrumbs. Stir in caster sugar.
Add the egg yolk and mix in. Bring together with your hands, pressing the ingredients together to make a ball. Turn on to the work surface and knead gently until smooth and well combined.
Preheat the oven to 200°C/400°F/Gas Mark 6. Thinly roll out the pastry on a lightly floured surface and use to line a 23cm (9inch) round, loose base tart tin, trimming off the excess around the edges. Line with greaseproof paper and fill with baking beans (or use dried beans reserved for the purpose).
Bake blind for 20 minutes. Remove the beans and paper and bake the pastry for a further 5 minutes.
Beat the eggs with 100ml water and pour into a saucepan. Add the sugar, lemon zest, lemon juice and butter. Heat gently, stirring, until the butter has melted and the sugar dissolved. Strain through a sieve into the pastry case and carefully transfer to the oven. Bake for about 20 minutes until the filling is lightly set.
Make up the Egg White Powder following the instructions on pack. Beat with a hand held electric whisk until softly peaking. Gradually beat in the sugar, a tablespoonful at a time, beating well between each addition until the meringue is thick and glossy.
Spread the meringue over the lemon filling, peaking it lightly with a palette knife. Return to the oven for a few minutes until the meringue peaks are golden. Watch very closely as the meringue with brown very quickly. Leave to cool before serving.