Put the flour, icing sugar, butter and 1/2 tsp salt in a food processor, and blend until combined. Add the egg yolks and 2 tsp cold water, then blitz again until the dough starts to form clumps around the blade. Tip onto your work surface and knead briefly to bring the dough together. Remove about a third of the dough, shape both pieces into discs and wrap. Chill for at least 30 mins. Alternatively, mix the flour, sugar and salt in a bowl and work in the butter with your fingertips before adding the yolks and water.

Mix the custard powder and 75g of the sugar with 100ml of the milk to make a smooth paste. Warm the rest of the milk in a saucepan. Pour the warm milk over the custard powder mix, stir and pour back into the pan with the vanilla. Heat until you have a really thick custard. Transfer to a bowl, cover to prevent a skin forming, and chill for at least 30 mins or until cold.

Heat oven to 200C/180C fan/gas 6 and put a baking sheet on the middle shelf. Toss the rhubarb, cornflour and remaining 125g sugar together in a bowl. Roll out the larger of the two pieces of dough on a lightly floured surface, to a rectangle large enough to line the base and sides of a 25 x 35cm shallow baking tin, about 1/2cm thick. Lift the dough over your rolling pin and drape into the tin, leaving any excess hanging over the edges. Stir the set custard and spread over the pastry base. Top with the rhubarb, in neat lines or randomly, and scatter any sugary bits over the top.

Roll out the smaller piece of pastry until large enough to cover the top of the pie. You can either cover the pie completely, leaving a few holes for the steam to escape, or create a decorative topping. When you’re happy with your design, brush the top with beaten egg and scatter over the 1 tbsp caster sugar. Place the tin on the tray on the middle shelf and bake for 40-45 mins, until the pastry is deep golden brown and the filling is bubbling. This pie is best served at room temperature or just warm, so leave to cool for at least 1 1/2 hrs before slicing.