Sift the flour onto your work surface. Mix in the salt with your fingertips, then make a well in the center of the mound of flour.
Mix the water and vinegar together and set aside.
Make a well in the flour and pour the melted butter in the well. Work it into the flour mixture, slowly adding a little of the water at a time until a dough forms (you may not need to use all the water as you don’t want the dough to be too wet!).
Form the dough into a square, cut a slit in the middle of one side, wrap and refrigerate the dough for 20 minutes.
Meanwhile, shape the 400 grams of dough into a perfect flat square with the help of a rolling pin. Wrap it and refrigerate until it is cold.
Take the chilled dough and roll it into a square (using just a little flour on the work surface and rolling pin to prevent sticking).
Place the square of butter in the middle (with the points of the square of the dough pointing towards the middle of the sides of the square of dough). Bring up the corners of the dough toward the center and pinch all the edges together. Basically you want to wrap the butter with the dough and seal it in tightly.
Roll the square into a rectangle for which the length is three times the width. Do a simple turn (folding in thirds like a letter). Flip the dough over, turn it 90 degrees, and repeat the rolling and simple turn. You’ve now made two simple turns.
Chill the dough for at least 1 hour (or overnight). Then roll it out and repeat the simple turn two more times. That makes four turns so far.
Chill the dough for at least 1 hour (or overnight). Then roll it out and repeat the simple turn two more times. Now you have done six simple turns overall and the dough is ready to use once you have chilled it again for at least 1 hour.