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. If you don't have a sifter, just grab handfuls of flour and sift them through your fingers from 6 inches above the bowl OR whisk it all a few times, and your dry ingredients will be incorporated and aerated.
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. Form dough into a flat rectangle and chill, wrapped in plastic wrap, at least 1 hour. If you're in a hurry, 30 minutes in the back of the freezer will do.
If it's a warm day, I like to keep the dough I'm not currently working in the fridge. The butter can melt quickly and make the dough challenging to work with.
Cut dough into equal sized cubes (about 1+1/2 inches) and roll into balls. Leave 3 or 4 out to roll, and put rest in fridge.
Roll into 4"-5" discs, depending on how big you want to make your empanadas.
