Sift the all-purpose flour, cornstarch, sugar, baking powder and salt together into a large mixing bowl. Make a well in the center of the dry mix.
Shred 1 cup of unsalted butter (2 sticks) into the well you created in the dry ingredients. Use your hands to toss the flour and shredded butter together.
Pour the cold milk into the butter-flour mixture and knead just until a thick dough forms.
Turn the dough out onto a floured surface and fold the dough over itself four times to develop a stiff dough.
Use a rolling pin to roll the dough out into a rectangle that’s approximately 12” long by 8” wide. Shred the remaining ½ cup of butter (1 stick) onto 2/3rds of the rectangle’s surface.
To encase the butter in the dough, first fold the unbuttered third of dough over the centered third- as if you’re folding a letter- then, fold the exposed third over the middle fold. You should have a rectangle with no buttered parts exposed.
Roll the dough out into a rectangle the same size as before. Fold the two short ends towards the center, then fold the dough over- as if you’re closing a book. Wrap the rectangle of dough in plastic wrap and chill the dough in the refrigerator for 60 minutes.
Remove the dough from the plastic wrap and re-roll to a rectangle, once more. Fold the dough in four parts as before. Re-cover the dough and chill for 40-60 minutes. This step may be done a day ahead.