Place the butter, flour, potato starch, salt and sugar in the bowl of a food processor and process until it has the consistency of sand.
Combine the milk and egg yolk in a small bowl. Add to the flour mixture and stir by hand until the mixture is evenly moistened. Do not overwork the pastry. Wrap it in cling film and place it in the refrigerator for at least 30 minutes.
Grease and flour a 24cm springform tin.
Make the fillling:
Preheat the oven to 210 C / Gas 6-7.
Pour the milk, half of the sugar and the split and scraped vanilla bean into a large heavy-based stainless steel saucepan over medium heat. Bring to the boil, stirring occasionally, to dissolve the sugar.
While the milk is heating, take a large bowl and whisk the eggs with the rest of the sugar until well combined. Stir in the cornflour and cream and whisk until well blended.
Once the milk is boiling, remove from the heat and let cool for a few minutes before gradually pour it into the egg mixture, stirring constantly.
Pour everything back into the saucepan and bring to the boil over medium heat, never stopping the whisking. As soon as it begins to thicken, remove it from the heat and whisk constantly for a few minutes while it cools.
Take the pastry out of the refrigerator and roll it out on a lightly floured work surface in a circle large enough to cover the bottom and sides of the pan. Pierce the pastry several times with a fork and place in the refrigerator until the filling is ready.
Once the cream has cooled, beat it with a hand whisk to smoothen it. Pour it over the pastry and even out the surface with a spatula.
Bake in the preheated oven for 20 minutes. Increase the temperature to 240 C / Gas 9 and bake for a further 10 minutes, keeping an eye on the custard during these last minutes so that it does not burn. To speed up browning the top you can also place it in an oven shelf closer to the top.
Let the flan to cool then place it in the refrigerator for 4 hours before serving.
To unmould, run the blade of a knife along the inside of the mold, to loosen the flan. Then carefully remove the ring off rom the springform pan.