Pie Crust: In a mixer fitted with a paddle attachment (or using a hand mixer), mix the flour, salt, and sugar for 1 minute. Add the butter and mix just until you have a crumbly, sandy mixture. You should still be able to see the pieces of butter. In a small bowl, stir the water and vinegar together. With the mixer running at medium speed, drizzle in the water-vinegar mixture and mix just until a dough forms. You should still see small bits of butter. Turn out onto a work surface, divide the dough in half, and shape into round, flat disks. Wrap separately in plastic wrap and refrigerate at least 30 minutes before using. (Or, refrigerate up to 48 hours or freeze up to 1 month before using. If frozen, let thaw in the refrigerator overnight before rolling out.)
On a floured surface, roll out 1 disk of dough into a square large enough to cut 4 circles about 6 inches in diameter (the dough will be about 3/16-inch thick). Using an individual (4-inch) pie pan as a guide, and leaving an extra inch of dough all the way around, cut the dough into circles with the tip of a sharp knife. Line the individual pie pans with the dough circles, then crimp the edges decoratively. Repeat with the remaining dough to make a total of 8 individual pie shells. Arrange the lined pie pans on a sheet pan and refrigerate, uncovered, 30 to 60 minutes.
Preheat the oven to 400 degrees F.
In a medium bowl, mix the flour, brown sugar, 1 tablespoon of the granulated sugar, and the cinnamon. Add the butter and, using your fingertips, pinch the ingredients together into a sandy, crumbly mixture. Do not overmix; as soon as the mixture is sandy, cover, and refrigerate until ready to use.
In a large bowl, toss the rhubarb, the remaining 1/2 cup sugar, and cornstarch together. Divide among the pie shells, mounding the filling high (it will cook down during baking). Sprinkle the topping over the pies, gently pressing the topping onto the rhubarb with your hand. Bake until the rhubarb is tender and the topping is golden brown, about 30 minutes. Serve warm or at room temperature.