4ozcold unsalted buttercut into 1/2-inch cubes (8 tbsp)
1tablespoondistilled white vinegar
Sift flour with salt into a large bowl and blend in butter with pastry blender until mixture resembles coarse meal with some (roughly pea-size) butter lumps. You can also do this in a food processor - pulse the flour and salt with the butter about 9 or 10 times.
Beat together egg, water, and vinegar in a small bowl with a fork. Add to flour mixture, stirring with fork until just incorporated. Don’t over-mix. If using a food processor, pour the egg mixture through the feed tube with the processor running until it just comes together.
Turn out mixture onto large piece of plastic wrap. Using the edges of the wrap, press dough together into a round ball. Flatten into a disc and chill, wrapped in plastic wrap, at least 1 hour, or freeze for up to 3 months.
Divide dough into 2 or 4 pieces. Working with one section at a time (keep the remaining dough covered so it doesn’t dry out), roll out dough on a lightly floured surface till very thin, about 1/16 of an inch. Using a round cutter about 4 1/2 to 5 inches in diameter, cut out as many circles as you can from the piece of dough. Place rounds on a plate and cover loosely with a damp cloth or plastic. (If you're freezing the individual tapas, place a piece of parchment in between each one.) Gather up remaining dough to roll out again. You don’t want to re-use the dough too many times, as the butter pieces will melt into the dough more and more, but you can re-roll a couple of times.
Repeat the process with the remaining dough. You should end up with about 25-30 rounds.