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

For the crust and topping, in a large bowl, whisk together the flour, 1/2 cup granulated sugar, and salt. Cut in the cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Press about two-thirds of this mixture evenly into the bottom of the prepared baking pan. Reserve the remaining one-third for the topping.

Bake the crust for 15 minutes, or until lightly golden brown. While the crust is baking, prepare the filling.

For the rhubarb filling, in a separate medium bowl, combine the chopped rhubarb, 1 cup granulated sugar, cornstarch, vanilla extract, and lemon juice. Mix well to ensure the rhubarb is evenly coated.

Once the crust is pre-baked, carefully spread the rhubarb filling evenly over the hot crust. Crumble the reserved flour-butter mixture over the rhubarb filling.

Return the pan to the oven and bake for an additional 30-35 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender when pierced with a fork.

Remove from the oven and let the bars cool completely on a wire rack before cutting into squares. This is crucial for clean cuts, as the filling needs to set. Enjoy!