The bars are crunchy-bottomed with that deep flavor of oats caramelized in butter and brown sugar, and then they're topped with a jammy rhubarb layer and a few tender crumbs. The rhubarb is, as it always is if you use enough sugar, both achingly sweet and jaw-crampingly tart, and the bars are so perfectly what I had in mind that I had to keep saying immodestly to our friends, "Aren't these so good?" And yes, they agreed happily, they are.
Heat the oven to 400 and heavily grease a lasagna-sized (11- by 7-inch) baking dish.
n a large mixing bowl combine the flour, oats, brown sugar, and salt. Now add the pieces of butter and toss to coat them with the flour mixture, then use your fingertips to rub the butter into the dry ingredients. This is a messy but not unpleasant job: you'll be lifting handfuls of the mixture up out of the bowl, then gently letting it fall through your fingertips as you rub it together. Eventually, you'll have a bowl full of pebbly crumbs, which is what you're going for.
Reserve a heaping cup of crumbs, and press the remaining crumbs into the baking dish, patting them down firmly to form a bottom crust. Spread the sliced rhubarb evenly over the crust.
in a small saucepan combine the white sugar and cornstarch; stir in the water and cook the mixture over medium heat, stirring constantly, until it turns clear and thickens slightly--around five minutes. It will foam up and it will never get especially thick, but this seems to be okay.
Remove it from the heat, stir in the vanilla, and pour it evenly over the rhubarb. Sprinkle with the reserved crumbs and bake the pan for ten minutes at 400 before turning the temperature down to 325 and baking for another 40 minutes.
Serve in squares, warm or at room temperature, with or without whipped cream or vanilla ice cream.