Place the bread flour, salt, cold water, and melted butter into the bowl of your stand mixer with the paddle attachment. Mix on low speed until everything is just combined. I'm using a high gluten flour (bread flour) to add more structure to the dough, you can also use all-purpose flour.
Now add your cold, diced butter. Make sure to not over mix, we want to see pieces of butter in the dough (this will create the flakiness we want).
Use your hands to knead your dough just a little bit to combine it all together.
Wrap the dough in plastic wrap and refrigerate for at least 2 hours.
Once your dough is chilled, flour your table, take the dough out of the fridge and roll it out in a long rectangle that is about 24" long and 8" wide.
Give it the first book fold by folding each end towards the middle making sure the ends meet.
Do your second book fold by folding the dough in half along the seam and closing the ends like a book. (Hence the term book fold.) Turn it a quarter of a turn.
Repeat the same process once more by rolling out your dough to do a second book fold. If your dough is bouncing back at the edges and is not rolling out easily, your gluten needs to relax. Wrap the dough and put it back in the refrigerator for an hour and try again. Also if your dough is too soft and warm, put your dough back in the fridge for 30 minutes.
Once your second book fold is completed, wrap and rest your dough for 30 to 60 minutes in the fridge.
Once your dough is chilled, cut it in half. This recipe will make 8 mille-feuille, but most sheet pans will only be able to fit enough puff pastry dough for 4 at a time.
With your first half of the dough, flour your table and roll the dough to the size of the sheet pan you will use to bake on. I'm using a 9"x13" sheet pan and am rolling the dough to ⅛" thick.
Trim the edges of your dough with a knife. Make sure to cut straight up and down so that it doesn't seal the edges of the layers.
Using your rolling pin, roll your dough onto the back side of your sheet pan upside down.
Use a docker or a fork to poke small holes in the dough to avoid the dough from over developing in the oven.
Place a piece of parchment paper on top of the dough, then place another sheet pan right side up on top. This will make sure your dough remains flat in the oven.
Bake at 360°F for 20-30 minutes until the edges are golden brown. While your puff pastry is cooling, make your pastry cream.