Gently melt the 85g butter in a saucepan and let it cool.
Sift the flour into the bowl of a stand mixer and add the water, butter and fleur de sel.
Beat at a low speed with a dough hook attachment until the dough is smooth and fairly firm, this will take around 3 minutes.
Flatten the dough into a square shape, cover in cling-film and leave to rest for at least 2 hours.
Take the remaining 335g of butter and place it between two sheets of greaseproof paper, roll the butter into a square around 13/15cm or a thickness of about 1cm. Wrap the butter in greaseproof paper and place it back in the fridge.
Roll the dough out into a square about 1cm thick, arrange the rolled out butter in the middle so that it looks like a diamond.
Fold the corners of the pastry into the centre of the butter like an envelope and make sure all of the butter is covered.
Run the rolling pin over the edges to smooth the surface, then roll the dough into a rectangle about 8-9mm thick, fold the top edge down two thirds, fold the bottom edge up to cover the top, so that it mimics a letter.
Cover the pastry in the cling-film and return to the fridge for 10 minutes.
Take the pastry out of the fridge and arrange it on a floured surface with an open edge facing you, roll the dough into a rectangle about 8-9mm thick.
Fold the dough into thirds like a letter and place it back in the fridge for 2 hours.
Repeat the last two steps twice more at 2 hour intervals, rolling out the dough and folding it like a letter.
To make the creme patissiere, split the vanilla bean lengthwise and scrape out the seeds.
Bring the milk, vanilla pod and seeds or extract to a boil in a saucepan over a medium heat.
Remove from the heat and let infuse for 10 minutes.
Meanwhile combine the sugar and cornstarch in a medium bowl. Add the egg yolks.
Whisk just until the sugar is combined, do not let it get too pale.
Whisk a third of the milk into the egg mixture to loosen it.
Whisk in the remaining milk and return the mixture to the saucepan, straining it through a fine sieve and cook over a medium heat, whisking constantly.
As soon as the pastry cream begins to thicken, remove it from the heat.
Whisk in the butter until smooth.
Scrape the pastry cream into a bowl and cover with a layer of clingfilm, pressing it down so it touches the surface of the cream. This will stop a skin forming.
Allow to chill completely.
Preheat the oven to 180C Fan, trim the rhubarb to lengths no less than 30cm (where possible) toss the rhubarb in the caster sugar and place in a small baking dish or roasting tray, cover it with foil and place it in the oven on a shelf for 25 minutes. The rhubarb should be tender but not mushy. Leave to cool.
***COOK THE PASTRY***
Line two baking trays with greaseproof paper. You will need 4 trays in total because a tray needs to be set on the dough to stop it puffing up too much whilst cooking.
Weigh the dough and cut off 400g, put the rest in the freezer and use it at a later date (it will last up to 3 months).
Roll out the remaining dough into a rectangle 40 by 30cm. Roll out the dough evenly without trying to reach the desired length in one go. Then roll crosswise , alternate the rolling until the desired dimensions are achieved. If the dough seems elastic give it time to settle on the worktop for a few minutes before rolling it out again.
Cut the pastry into three 30 x 13cm lengths, use a rolling pin to transfer the pastry to the prepared baking sheets. Prick the pastry all over with a fork and sprinkle it with sugar to help it caramelise.
Lay a piece of greaseproof paper on top of the pastry and sit a tray of the same size on top. Place the pastry in the oven and cook for 20-25mins until the pastry is golden. You may need to rotate the pastry during cooking.
When cooked, take the trays and the greaseproof paper off the pastry and leave to cool on the trays for a few minutes, before resting the pastry on a cooling rack.
***ASSEMBLE THE MILLE-FEUILLE***
Take the bottom layer of pastry and place it on a piece of greaseproof paper (this will make clean up easier), Cut the rhubarb into lengths around 3.75cm long and lay them end to end in four lines on the bottom layer of the pastry, with space between the lines to allow for the creme patisserie. The reason behind this is that the rhubarb doesn't cut well, this will make cutting the finished pastry much easier.
Pipe 5 lines of creme patissiere on the bottom layer, making sure this makes up the sides and piping in between the rhubarb.
Lay another piece of pastry on top and then repeat with the remaining rhubarb cut into 3.75cm lengths and the remaining creme patissiere.
Lay the final layer of pastry on top and prepare the icing to decorate the top. Sift the fondant icing sugar and add 4 Tbsp of the reserved liquid from the rhubarb, if you don't have this much liquid just use water. Stir the icing until it is the text of thick cream and everything is combined.
Take some of the icing and place it in a small bowl, add some yellow food colouring to this icing and mix well. Put this icing in a piping bag with a very thin nozzle, or use a disposable bag and cut a very thin nozzle.
Add pink (or a small amount of red) food colouring to the remaining icing and stir well to combine. Pour the pink icing over the top of the mille-feuille and use a palette knife to spread the icing evenly.
Take the yellow icing in the piping bag and pipe long thin lines across the length of the pastry. Using a cocktail stick, pull it back and forth through the icing to create the desired feathering on the cake.
Leave the icing to set for around 15-20mins before attempting to cut it. Using a serrated pastry knife try to line the knife up with your lengths of rhubarb and cut the pastries into 3.75cm sized slices. The rhubarb doesn't cut well with the serrated knife, so that is why the pieces of rhubarb were trimmed.
The slices will need to be kept in a fridge and need to be eaten within 48 hours before the pastry gets too soggy or the fondat icing begins to melt.