Make Royal icing; dilute some by whisking in milk to create flood icing
Roll out puff pastry
Cut slices for napoleon; either several big napoleons or smaller slices for individually-sized servings.
Place onto parchment paper
Cover with a second sheet of parchment and a heavy pan to keep puff pastry from "puffing" during baking
Bake for about 11 minutes at 400*F rotating the pan halfway through.
Cool puff pastry completely. Use a sharp knife to square the edges off after baking for a perfect edge.
Assemble ingredients - chilled patissier cream, fillings, royal icing, chocolate ganache or fruit glaze, etc.
Layer each piece of puff pastry with pastry cream in an even layer from edge to edge. If adding additional fillings, place them on top of the pastry cream layer.
Continue layering Mille-Feuille
Use thick royal icing to trace edge. Use flood icing to cover center of the Mille-Feuille
While the royal icing is still wet, use a pastry bag and apply thin lines of chocolate ganache or glaze
Use a toothpick or the tip of a paring knife and pull through the royal icing in alternating directions.