How to Make It
Combine 1/2 cup hot water, brown sugar, vinegar, and salt in a small metal bowl. Stir until sugar dissolves. Place liquid in freezer until very cold, about 20 minutes. Transfer mixture to refrigerator, and chill until ready to use.
Place flour and 1/2 cup cubed butter in a large bowl. Pinch and smear butter between your fingers. (Processing butter like this creates small leaves of butter that layer in the dough, resulting in a flaky crust.) Once all butter chunks have been pinched, grab small handfuls of flour and butter, and rub them together between your palms until mixture resembles uneven pebbles on a sandy beach.
Dump crumbly mixture out onto a clean work surface. Scatter remaining 1/2 cup cubed butter over dough. Use palm of your hand to smear butter as if you were sliding a secret message across a table. Use a bench scraper to gather shaggy dough, and repeat smearing process until you have a pile of striated rubble. Larger pieces of butter will create a marbled dough and will melt during baking, causing water in butter to evaporate, resulting in flaky pastry pockets.
Gather dough up in a mound, and form a well in center. Remove cold liquid mixture from refrigerator, and pour half into well. Using fingertips, slowly bring dough into center, combining with liquid, until liquid has been incorporated. Gather moistened dough into a shaggy pile.
Slowly pour remaining half of liquid onto pile of dough. Lift dough from bottom, and squeeze just until it comes together into a mass. Divide dough ball in half (about 12 ounces each), and shape each half into a disk. Wrap each disk in plastic wrap, and chill in refrigerator 2 hours before using.