Sift the flour with the salt and sugar on to a board and make a well in the center. Crumble or sprinkle the yeast over half the warm water in a bowl and leave until dissolved.
Pour the yeast mixture into the well with the milk and remaining water and mix gradually, drawing in the flour with a spatula or pastry scraper to make a smooth dough. Work just until the dough is mixed, adding more flour if it is very sticky. Shape it into a ball and let rest in the refrigerator 15 minutes.
Sprinkle the butter with flour and, with a rolling pin, pound it until flat. Fold it and continue pounding and folding until it is pliable. Shape the butter into an 8in/20cm square. Roll the dough into a 16in/40cm square, put the butter in the center, and fold the dough to enclose it, pinching the edges to seal the package.
Flatten the package of dough and butter slightly with the rolling pin and turn it over. Roll out the package to a 24in/60cm x 8in/20cm rectangle.
Fold over one of the short sides of the dough, then the other, so that the rectangle is folded in three layers, like a business letter, and forms a square. Press the open ends lightly with the rolling pin to seal them.
Give the dough two turns. Wrap it and let it rest in the refrigerator 20-30 minutes until firm. Give it two more turns; rest again 20-30 minutes.