In a large bowl, whisk together flour and salt. Toss in cold butter cubes, coating them lightly with flour. You want pea-sized bits plus some larger chunks—those create flakiness.

Drizzle ice water (and vinegar, if using) over the flour-butter mix. Use a fork or pastry cutter to bring the dough together—just until it holds. Don’t overmix; bits of butter visible are good.

Turn the shaggy dough onto a lightly floured surface. Gently press into a rough rectangle, about 1-inch thick. Wrap in plastic and chill 20–30 minutes—this firms the butter and relaxes the flour.

On a floured board, roll dough into a 10×18-inch rectangle. Fold the top third down, then the bottom third up (like a business letter). Rotate 90° so folded edge is left, chill 15 minutes.

Perform two more letter folds, chilling 15 minutes between each. Aim for 6–8 layers total. If dough feels sticky, flash-chill 10 more minutes.

After the last fold, wrap and chill at least 1 hour (or overnight). When ready, roll out to desired thickness (⅛–¼ inch) for tarts, croissants, turnovers, or palmiers.