1 cup plain flour
¾ cup icing sugar mixture
250g cold unsalted butter, grated
4 free-range eggs
3 free-range egg yolks
1 cup caster sugar
1 Tbsp cornflour
Finely grated zest and juice of 4 lemons
Finely grated zest and juice of 2 grapefruits
1 tsp vanilla bean paste
Preheat oven to 180 degrees C fan-forced (200 degrees C conventional). Sift flour and icing sugar into a large bowl, then add half the butter and rub with fingertips until crumbly. Add ¼ cup iced water and mix to form a shaggy dough. Press into bottom of a lined 27 x 17cm slice pan, prick with fork and then bake for 18-20 minutes.
Mix eggs, yolks, caster sugar, cornflour, zests, juices and vanilla in a bowl and whisk until smooth. Transfer to a medium saucepan set over low-medium heat and cook until barely simmering. Remove from heat and whisk in remaining butter.
Strain through a fine sieve over baked base, them bake for 20-25 minutes, until firm. Allow to chill completely before slicing. Serve.