You'll have to discard any scraps, since this dough can't be rolled again.
Preheat oven to 325 degrees. In the bowl of a food processor, pulse flour, salt, and sugar until combined. Add butter pieces, and pulse until mixture resembles coarse meal. With the machine running, gradually add milk; process until dough comes together. Cover dough with plastic wrap, and allow to relax at room temperature for 15 to 20 minutes.
Unwrap dough, and transfer it to a lightly floured surface. Roll into 17 1/2-by-17 1/2-inch square, 1/16 inch thick (if dough continues to spring back, allow to relax again, covered with plastic wrap, for another 10 minutes). Sprinkle dough lightly with coarse salt. Gently press salt into dough with the rolling pin.
Using a knife or pizza cutter, cut dough into 2 1/2-inch squares. With a thin spatula, transfer to ungreased baking sheets. Liberally pierce with fork. Bake for 20 to 25 minutes, until lightly browned and crisp, rotating baking sheets if needed for even cooking. Let cool on wire rack. Keep crackers in an airtight container, at room temperature, about 1 week.