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Whisk 4 egg yolks with 125g icing sugar and the vanilla sugar until light and fluffy. Scald the orange with hot water and grate off the zest.
Sift the plain flour and potato flour and tip them onto a pastry board or lightly floured worksurface, together with the baking powder. Grate over 250g of the butter and chop it through the flour with a knife and then rub it in to make the mixture more like crumbs. Pour on the yolk mixture and orange zest and bring the mixture together, then knead to a soft dough.
Cut off a third of the dough, wrap it tightly and put it in the fridge. Roll out the remaining dough and use it to line the base and sides of a 26cm springform pan that you have already lined with a circle of buttered baking parchment. Put the tin in the fridge for 20 minutes. Heat the oven to 180C/fan 160C/gas 4. Line the dough with baking parchment and fill with baking beans to keep the sides in place, then bake the base for 10-15 minutes.
To make the filling, push the curd cheese through a metal strainer. Beat the remaining butter until light and fluffy, add the cheese, 8 egg yolks, 80g icing sugar, the pudding powder and orange extract. Mix for about 2 minutes until fluffy. Beat 2 egg whites with a pinch of salt until stiff and beat in the remaining icing sugar to make a stiff meringue mixture. Fold the egg whites into the curd mixture.
Beat the remaining 3 egg whites with the caster sugar until meringue-like. Unwrap the raw dough and grate it on a large-tooth grater. Spread the cheese mixture into the baked base. Drain the peaches, cut them into smaller pieces and put them on top of the cheese filling. Cover the dough with meringue, then sprinkle a little grated pastry on top. Knead the remaining dough, roll it out, cut out thin strips and arrange them in a lattice pattern. Bake for 50 mins-1 hr. Cool in the turned off oven for 40 mins before removing. Cool completely.