Step 1 Place the flour, salt and sugar in the bowl of a food processor and pulse a few times to combine. Cut the butter into small pieces and drop them into the flour mixture. Process a few seconds, until the mixture resembles coarse meal. (If you don't have a food processor, you may do this using a pastry blender or two forks.)
Step 2 Combine the milk and egg and add to the flour mixture. Pulse 5 times, then process until the dough forms into a large ball. (If you do this by hand, make a well in the middle of the flour, add the egg and milk and use a wooden spoon to combine, then knead until smooth.) Wrap the pastry in a cloth or plastic wrap and chill for at least 2 hours.
Step 3Roll out the dough on a lightly floured board to a one-eighth inch thickness and cut out rounds with a 2 1/4 -inch round biscuit cutter. Use scraps to form another ball, roll out again, and cut out more rounds.
Step 4Push a round into a 2 1/4 -inch tart tin, pressing it gently into the bottom and corners. Repeat with the remaining dough, working in batches if necessary. Makes about 40 (2 1/4 -inch) tart shells.