For a millefeuille, puff pastry must bake flat rather than billowing upward. Place a cooling rack directly on top of the pastry sheet before it goes into the oven - the weight provides gentle, even pressure without trapping steam, so the pastry still cooks through and crisps up, but stays compact enough to stack cleanly. Some chefs use a second baking tray on top for even more control. Whichever method you use, the goal is the same: uniform, flat sheets with a consistent height across each layer, which is what gives a millefeuille its elegant, precise appearance.