In a mixing bowl, combine the 1 cup flour and the powdered sugar. Cut in butter until mixture resembles coarse crumbs.
Pat the crumb mixture into the bottom of an 11x7x1-1/2-inch or 9x9x2-inch baking pan. Bake in a 350 degree F oven for 12 minutes.
In the mixing bowl, beat together granulated sugar, the 1/4 cup flour, the eggs, and vanilla. Stir in rhubarb. Pour over warm crust in baking pan.
Bake for 35 minutes more or until done. Let cool in the pan on a wire rack. Serve warm or cool. Store in the refrigerator. Makes 16 servings.
If using frozen rhubarb, thaw just enough so fruit will chop easily. Then use the rhubarb as directed.