For the dough: Mix the milk and water in a medium bowl to combine and set aside. Then put the flours, yeast, sugar and salt in a bowl of a food processor and pulse to combine. Dice 125g of the butter into small pieces and add to the food processor. Then pulse once or twice just to mix together. Don't overmix at this stage! You still want to be able to see chunks of butter.
Tip the butter and flour mixture into the liquids and using a spatula, fold together whilst still maintaining the chunks of butter. Once you have formed a rough dough, tip this out onto a lightly floured surface and work very lightly into a ball of dough. Form into a flat rectangle, wrap in clingfilm and put in the fridge for 45 minutes. Meanwhile, put the remaining butter into the freezer to chill it thoroughly.
Remove the dough from the fridge and roll out into a long rectangle three times as long as it is wide - roughly 15x45cm. Take the butter out of the freezer and and coarsely grate it over the bottom two-thirds of the dough. Fold the top third of the dough over the middle-third, then fold the bottom third over the other-two thirds as if folding a letter. This is your first turn. Wrap the dough in clingfilm and chill in the fridge for 20 minutes.
Remove the dough from the fridge and turn the dough 90 degrees so that the open ends are facing you. Then repeat the rolling and folding process twice more, giving the dough a total of three turns. Wrap the finished dough in clingfilm and put in the fridge overnight before baking.
For the creme patissiere: Place either the seeds of vanilla pod or the vanilla bean paste into a medium saucepan. Pour over the milk and bring to the boil over a medium-high heat.
Meanwhile, put the egg and yolks in a heatproof bowl and whisk in the sugar and cornflour until smooth. Pour the boiling milk over, whisking constantly to combine. Pour this mixture back into the pan and return to a medium heat. Whisk constantly until thickened, cooking for a few minutes extra to remove the taste of the cornflour.
Pour the creme patissiere into a clean bowl and press a piece of clingfilm onto the surface to stop it forming a skin. Then pop into the fridge to chill overnight.
Put the raisins into a small bowl, cover with boiling water and set aside for 30 minutes. This plumps up the fruit and stops them from burning as they bake.
Line a large baking tray with parchment and set aside. Roll out the dough on a lightly floured surface into a rectangle about 24x48cm with the short edge facing you, then trim to square off the edges. Spread the creme patissiere across the pastry leaving a clear border of 2.5cm along one far edge.
Drain the raisins and sprinkle over the filling. Brush the beaten egg down the 2.5cm border, and then roll up the dough into a tight swirl using the egg washed border to stick the dough together. Cut into 16 equal slices and place onto the prepared baking tray. Lightly cover the tray with clingfilm, and leave to prove in a warm place for 2-3 hours until almost doubled in size and puffy to the touch.
Preheat the oven to 180C/160C Fan/Gas Mark 4. Brush the beaten egg over the top of the pastries, then place in the oven to bake for 20-25 minutes until golden brown. Remove from the oven and leave to cool on the trays for 5 minutes, before transferring them to a wire rack to cool completely.