Pour 250 ml of milk in a jug and add the flour. If you do not have flour, cornflour (corn starch) can be used instead.
Whisk until smooth, or pass through a sieve if there are any lumps formed.
Add the other 250 ml of milk to a pan together with the sugar, and simmer over a low heat for 2-3 minutes, then add the milk and flour mixture and whisk well.
Add the egg yolks and vanilla extract and continue to whisk until the cream thickens well.
Remove from the heat, leave to cool slightly, and add the butter cut into small cubes.
Mix well until the butter is fully incorporated and set aside to cool completely in the fridge.
Pour the whipping cream in a large bowl and beat it with an electric mixer until it forms stiff peaks.
Preheat the oven to 200 degrees C. (398 Fahrenheit).
Place the ready rolled puff pastry on the back of a baking tray, prick it with a fork in a few places, and pop it in the oven for about 15 minutes or until golden brown.
Remove from the oven and leave it to cool.
Cut the puff pastry in half. Place one half on a large plate and top it first with the cream, then with the whipped cream.
Crush the other half of the puff pastry and use the flakes to sprinkle over the cake.
Refrigerate for at least 4 hours, ideally overnight before slicing the cake.