Add butter and cut in with a pastry cutter, (or if you have a stand mixer, use the flat beater) until mixture resembles coarse crumbs.
Add water, 1 tablespoon at a time, mixing until the dough starts to hold together.
Divide dough in half, and form each half into a smooth ball. Wrap pieces separately in plastic wrap. Put one piece in the freezer for another time, and refrigerate the other for 20 minutes.
Remove dough from the fridge and plastic wrap. On a lightly floured surface, roll out the dough into a circle about ⅛ inch thick. Put the dough onto an 8-inch tart pan, pressing onto bottom and sides, and removing and overhanging dough.
Prick the bottom of the crust with a fork in several places.
Preheat oven to 375 degrees
Combine flour, sugar, and cinnamon, and sprinkle evenly over the bottom of the prepared tart shell. Place plum halves, skin side down, in a circular method to cover the entire crust.
Bake 45 minutes, or until plums are tender and crust in golden.