Combine flour, salt, and sugar in a food processor. Chop chilled butter into small cubes, add to flour mixture, and process 15-20 seconds, until it resembles fine gravel.
While running processor, stream 1/8 cup of ice water into the flour mixture, just until dough holds together when pinched. If necessary, add more water.
Dump dough out onto a work surface, gather into a disk shape. Wrap with plastic wrap, chill for 1 hour.
Lightly flour your work surface, roll chilled dough out pretty thin – 1/8″ to 1/4″, depending on your tastes – some prefer a thinner shell, some thicker. Cut 4″ rounds from the pastry – you’re aiming for 12.
Carefully transfer the pastry rounds to a muffin pan. I like to flatten the bottom against the tin, and work out from there, flattening the whole round to be flush with the muffin pan cavity – it holds the most filling! Feel free to get decorative about it – flattening the bottom of the dough against the muffin pan, gently ruffling the edges… it’s up to you! Chill the pan of prepared tart shells until ready to use.