Step 1Place 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 2Combine 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.
Step 1Heat the oven to 400 degrees. In the bowl of an electric mixer, mix the butter, sugar, maple syrup, eggs, salt, vinegar and vanilla. Stir in the walnuts. Stir the filling again as needed if it separates.
Step 2Spoon a scant 1 tablespoon of filling into each unbaked tart shell. Reduce the oven temperature to 375 degrees and bake the tarts for approximately 15 to 17 minutes.