Sift flour with salt into a large bowl and blend in butter with your fingertips or a pastry blender until mixture resembles coarse meal with some (roughly pea-size) butter lumps.
Beat together egg, water, and vinegar in a small bowl with a fork.
Add to flour mixture, stirring with fork until just incorporated. (Mixture will look shaggy.)
Turn out mixture onto a lightly floured surface and gather together, then knead gently with heel of your hand once or twice, just enough to bring dough together. (Deb note: If you use a large-ish bowl, you can do this step in-bowl.)
Form dough into two flat rectangles and chill them, each wrapped in plastic wrap, at least 1 hour. Dough can be chilled up to 6 hours total.