Preheat oven to 190C.
Sift the flour and custard powder into a bowl.
Rub in butter with fingertips until it resembles breadcrumbs.
Stir in sugar.
Mix in yolk and water with a knife to form a soft dough.
Cover with plastic wrap and refrigerate for 30 minutes.
Lightly grease a 20 cm pie plate.
Roll out pastry between 2 sheets of baking paper and line the pie plate. Trim the edge.
Line with crumpled baking paper, fill with beans or baking beads and bake for 10 minutes.
Remove paper and beads and bake another 5 minutes until the base is dry. Remove from oven and cool.
Reduce oven temperature to 160C.
Custard: Whisk the eggs, yolks, vanilla and sugar in a bowl. Add the cream and combine.
Pour into cooled pastry case and sprinkle with nutmeg.
Bake 50-55 minutes until set but not too firm.