To make the white chocolate cream, heat the milk and vanilla in a medium saucepan until just before boiling point (this is called scalding).
In the meantime, mix together the egg yolks, sugar, flours and pinch of salt until the mixture is smooth and thick.
Once the milk mixture is hot, pour it slowly into the egg yolk mixture whilst continuously whisking.
When all the milk has been incorporated, transfer the runny mixture back to the saucepan and over a gentle heat, allow to cook and thicken, whisking continuously.
Once the mixture is thick and there is no flouriness when you taste it, remove it from the heat and beat in the chopped white chocolate. Stir until the chocolate has melted and then pass the mixture through a sieve just in case there are any lumps.
Place in a bowl and put a piece of cling film directly onto the surface of the cream to prevent a skin from forming. Place in the fridge and allow to cool.
To make the pastry layers, start by pre-heating the oven to 180°c and line a baking sheet with parchment paper.
Roll the puff pastry out on a floured surface and cut into 8-12 rectangles of 5cmx10cm (depending on whether you want 2 or 3 layers). Place the rectangles onto the prepared baking sheet and sprinkle with caster sugar.
Place in the oven and bake for 10 minutes until the pastry is golden brown and puffed up.
Remove from the oven and allow to cool completely. To assemble the Mille-Feuille, layer the pastry rectangles with the white chocolate cream and berries. Dust with icing sugar and serve immediately.