Preheat oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.

Prepare the Crust/Crumble mixture: In a large bowl, whisk together 1 cup flour, oats, 1/2 cup granulated sugar, baking powder, and salt.

Cut in 1/2 cup of the cold, cubed butter until the mixture resembles coarse crumbs. Reserve 1 1/2 cups of this mixture for the topping layer, and press the remainder evenly into the bottom of the prepared baking pan to form the crust.

Prepare the Filling: In a separate bowl, gently toss the chopped rhubarb with the remaining 3/4 cup sugar, cornstarch, and vanilla extract until evenly coated. Spread this filling evenly over the crust layer.

Finish the Bars: Take the reserved 1 1/2 cups of crumb mixture and sprinkle it evenly over the rhubarb filling. Dot the top evenly with the remaining 1/2 cup of cold, cubed butter.

Bake for 45 to 50 minutes, or until the topping is golden brown and the rhubarb filling is bubbly. If the topping starts browning too quickly, loosely cover with foil.

Let the bars cool completely in the pan on a wire rack (at least 2 hours) before lifting out using the parchment overhang and cutting into 12 bars.