Cut the cold butter into 1/4" (6mm) square pieces. Place the butter in the freezer for at least five minutes.
Whisk the milk and apple cider vinegar together until it bubbles. It will thicken as it sets for five minutes. This is your buttermilk.
In a large bowl, mix the flour, sugar and salt. Using your hands (or a pastry cutter), cut in the butter by pinching/flattening the butter as you toss it with the flour. The butter should remain in larger chunks, such as almond size. Work quickly to keep the butter cold.
Pour in the milk mixture. Use your hands to incorporate the buttermilk, folding and pressing the mixture. Flattening out the butter is okay. To test hydration, grab a hand full of dough and squeeze it. If it holds together, it's ready for further shaping without adding more liquid. If it's still crumbly or falls apart, add a splash of milk or cold water and continue folding and pressing to a shaggy consistency. If the butter is starting to get a bit warm at this point, put the mixture in the freezer for about 5 minutes. We want the dough to stay as cold as possible.
On a lightly floured work surface, dump out the shaggy dough. Using a bench scraper, fold and press the dough at least 6-8 times, eventually shaping the mixture into a rough rectangle. Use the heel of your hand to press and schmear the dough onto the work surface. Gather the dough once again using the bench scraper and shape into a rectangle. Fold and press the dough over and on top of its self 7-8 more times. This helps to hydrate the dough and build flaky layers.
Gather all the bits together and shape into a rough disk approximately 1/2" (12mm) thick. Wrap tightly in parchment paper and rest in refrigerator for at least and hour, but preferably two if you can. MAKE AHEAD TIP: at this point the dough may be rested in the fridge for up to two days or frozen for up to two weeks. Wrap snugly in parchment, then refrigerate or freeze in a lidded container or freezer bag.