*The mille-feuille 'Napoleon' can be made with store-bought frozen puff pastry sheets. However, made with homemade inverted puff pastry will bring this pastry to the next level of flakiness and mouthfeel. (Look up for the inverted puff pastry recipe).
In a small saucepan, bring milk, some of the sugar and vanilla to a simmer. Meanwhile, combine yolks with remaining sugar, starch and flour. Temper yolk mixture with hot milk and bring back to the stove. Bring to boil and cook for 2 minutes whisking constantly. Transfer custard onto a frozen tray lined with plastic wrap. Top with another plastic sheet in contact; cool to room temp and chill completely. Beat heavy cream to medium-firm peaks and set aside. Beat chilled custard to smooth out and fold in whipped cream.
Roll out pastry into 11''x14''/28x35 cm and 1/4''/6mm thick rectangle. Chill to relax prior baking and prick all over. Place a wire rack on top of the pastry not in direct contact though but rather leaving some free space in between. Bake for about 45 minutes at 400ºF/200ºC.
Divide pastry into 3 rectangles and each into 6 or 8 equal portions. Pipe out diplomate cream. Dust out some powdered sugar. Enjoy!