Preheat oven to 375F degrees

Butter a 9 x 13 pan and line the bottom with parchment

For the filling:

1. In a medium saucepan, whisk together the cornstarch and sugar. Add in the rhubarb and water and stir until everything is well combined. Over medium heat, cook until the sugar is melted and soupy. Increase heat to medium-high and bring to a full boil. Stirring constantly, boil 1 - 2 minutes, until the the sauce is thickened, no longer cloudy, and the rhubarb has broken down. (The total time for making the filling is ~10 minutes.) Remove from the heat and stir in the vanilla. Set the filling aside while you make the bars.

For the bars:

1. In a large bowl, stir together all the ingredients except the butter. Rub in the cold butter with your fingers, a pastry blender, or a fork until the mixture looks pebbly.
2. To assemble, reserve 1 cup of the oat-flour mixture. Press the remaining mixture into the prepared pan. Top with the rhubarb filling (it will still be warm) and spread evenly. Then sprinkle with the reserved oat-flour mixture.
3. Bake 30-35 minutes, until golden brown and the edges are bubbling. Let cool completely before slicing. These bars are equally delicious at room temperature or cold out of the fridge.