In a food processor, add the butter to the flour mixture. Pulse until the mixture is crumbly, which should take about 30 seconds. With the processor running, gradually add 2/3 cup cold water in a slow, steady stream through the lid opening until the mixture forms a ball.
Flatten the dough into 2 disks, wrap them in plastic wrap, and refrigerate for at least 1 hour. While the dough chills, preheat your oven to 350 degrees.
On a lightly floured surface, roll out one pastry disk to a 1/8-inch thickness. Fit this dough into 12 (2-1/2-inch) mini pie plates and trim the dough around the edges. Freeze the pie shells until the dough is firm, about 10 minutes. Before baking, prick the bottoms and sides of the dough with a fork.