Preheat your oven to 400° F (200° C).
On a lightly floured surface, roll out thawed, but still chilled, puff pastry dough into an elongated rectangle. You want the rolled-out dough to be fairly thin – about the thickness of a thin piece of cardboard. Use a sharp knife to cut the dough into equal portions.
Give attention to how thin you roll the dough, as the first time I made mille-feuille I apparently rolled the dough a bit too thick. It looked fine, both before and after baking, but once everything was assembled and sliced, the finished mille-feuille was too high to bite if eaten by hand.