In a large bowl, combine the first four ingredients. Cut in butter until mixture resembles coarse crumbs. Press half of the mixture into a greased 8-in. square baking dish. Set remaining crumb mixture aside for topping. Sprinkle rhubarb over crust; set aside.
In a small saucepan, combine the sugar and cornstarch; gradually whisk in water until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened.
Remove from the heat; whisk in vanilla. Pour over rhubarb. Sprinkle with reserved crumb mixture.
Bake, uncovered, at 350° for 1 hour or until bubbly and lightly browned. Serve warm with ice cream if desired.
If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.