For the dough packet (détrempe): Knead the flour, salt, water and egg yolk together until smooth, cover and chill for about 30 minutes in the refrigerator.
For the butter packet (beurrage): Quickly knead the butter with the flour, form into a brick and chill for about 30 minutes in the refrigerator.
Place the dough packet on a lightly floured work surface and roll out into a 3 mm (approximately 1/8 inch) thick square, slightly thicker in the center.
Place the butter packet in the middle so that it looks like a diamond in the center of a square.
Using a rolling pin, roll the corners of the dough square out thinly.
Fold the dough corners over the butter packet then carefully roll out until 1/4-1/2 inch thick rectangle.
Make a letter fold: Fold 1/3 of the dough over the center 1/3 then fold the remaining 1/3 over top.
Rotate the dough 90° and roll out again into a rectangle. Make a book fold: Fold the 2 sides into the center to meet then fold again into a book. Always work as quickly as possible, so that the dough stays cool.
Wrap the dough in plastic wrap and chill for about 30 minutes in the refrigerator.
Roll out the dough into a rectangle and make another letter fold. Roll out again and make a book fold.
Cover with plastic wrap and chill in the refrigerator overnight before using.