For the dough:
3 cups (10 1/8 oz.) whole wheat pastry flour
3 cups (12 ¾ oz.) unbleached bread flour
2 tbsp. (½ oz.) nonfat dry milk
4 tbsp. (2 oz.) chilled unsalted butter, cut into small pieces
2 tsp. salt
1½ cups plus 2 tbsp. (13 oz.) water
For the butter square:
2 cups (1 lb.) unsalted butter, softened but still cool
1/3 cup (1 1/8 oz.) whole wheat pastry flour
Unbleached all-purpose flour, for dusting the work surface
In the bowl of an electric mixer, combine the flours and dry milk. Add the butter to the bowl and cut into the dry ingredients until the mixture resembles coarse meal and the largest pieces of butter are the size of peas. (This can also be done by hand with a pastry blender.) Mix the salt into the water, then add the water to the bowl with the flour mixture. Mix gently until a dough pulls together.
Turn the dough out onto a lightly floured surface and knead gently until it is smooth and a bit springy, about 2-3 minutes. (Avoid adding extra flour as much as possible – the more flour added, the more dense your dough will be.) The dough may be slightly wet or tacky initially because the whole wheat flour takes more time to absorb the liquid. Pat the dough into a square 1-inch thick, wrap with plastic wrap and refrigerate at least 1 hour.
To make the butter square, combine the butter and flour in a bowl. Mix until smooth and well blended with no lumps.
Lightly flour a piece of plastic wrap or wax paper, add the butter mixture to it and spread/pat into an 8-inch square. Cover, place on a flat surface in the refrigerator and chill for at least 30 minutes.
Remove the dough from the refrigerator and place on a lightly floured work surface.
Gently roll into a 12 inch square. Place the butter square on top in the center of the dough at a 45˚ angle.
Pull the flaps of the dough over the edges of the butter square until they meet in the middle. Pinch and seal the edges (moistening slightly if necessary.)
Dust the top of the dough with flour, them turn over and tap gently with the rolling pin into a rectangular shape. (Check the underside of the dough periodically to be sure it isn’t sticking, adding more flour if necessary.)
Roll the dough out into a 10 x 20-inch rectangle.
Brush any excess flour off the surface of the dough. Mark off the dough into thirds widthwise. Fold one side over and then the other on top, like a business letter, taking care to line up the edges carefully.
This completes the first turn.
If the dough is still cool and relaxed, roll it out again to a 10 x 20-inch rectangle and then fold into thirds. (If not, chill the dough 20 minutes before proceeding.) This completes the second turn.
Repeat this process four more times for a total of six turns. Wait at least 1 hour between sets of turns (so do 2 turns, wait 1 hour, 2 more turns, wait 1 hour, last 2 turns.) Wrap in plastic wrap and chill for 1 day, or freeze until ready to use.