Add the yeast and sugar to milk and give the mixture a quick stir. Allow 5 minutes for the yeast to bloom.
Mix the flour and salt. On a table or other clean surface, make a well with the flour and pour the milk into the well, gradually incorporating the flour into the milk until they are fully combined.
Knead for 15 minutes by hand. Chill and allow to rest for an hour. Meanwhile, make the butter packet.
Place your butter packet on one half of the dough, then dip a pastry brush into water, and brush around the edges. Encase the butter in the dough and press edges to seal. Chill for 20 minutes.
Using a dusting of flour, roll this out into a rectangle measuring 9 x 20 inches. Using a dry pastry brush, brush off any excess flour; otherwise, it will get trapped and start to brown, eventually discoloring your dough. Fold your dough into thirds and repeat (roll back out to 9 x 20, brush off excess flour, then fold into thirds). Chill for an hour.
Repeat this process 2 more times (roll, fold, roll, fold, chill), each time chilling the dough for 1 hour, for a total of six folds. It is helpful to make indentations with your finger on the dough to keep track of where you are with your folds.
When you're ready to use the dough, proof it in a warm place for an hour and bake at 425° F.
Cut the butter into 1/2-inch cubes and knead the flour into it. Once well combined, shape it into a 5-inch square -- flour your hands first so that they won’t stick to the butter.
Wrap the square in plastic wrap and chill for 30 minutes.