Meanwhile, in a large bowl, mix the next 3 cups flour with the sugar and salt.
Pour the pre ferment mixture, which has already doubled in size, into the flour and mix gently with a wooden spoon.
Add the milk and stir until the ingredients are combined.
Add the cubed butter and knead the dough until just smoothing out and slightly elastic. Knead for no more than 6-7 minutes. Do not knead the dough too much.
Transfer the dough to an oiled bowl and cover with plastic wrap.
Refrigerate for 1 hour or until the dough has doubled in size. Watch the dough and not the clock for this step.
On a large piece of parchment paper, cut the butter into half inch thick slices and shingle them, overlapping them as needed to make a 3 1/2 by 4 1/2 inch rectangle.
Cover with another piece of parchment paper and, using a rolling pin, roll out the butter into a rectangle no larger than 5x6 inches.
Leave it wrapped in parchment and refrigerate.