Preheat your 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 mixture: In a large bowl, whisk together the flour, 1/2 cup sugar, baking powder, and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs.

Press about two-thirds (2/3) of the crumb mixture evenly into the bottom of the prepared baking pan. Bake the base for 15 minutes until lightly golden.

While the base bakes, prepare the filling: In a medium bowl, gently toss the chopped rhubarb with 1/2 cup sugar, cornstarch, and vanilla extract until evenly coated.

Remove the par-baked crust from the oven. Spread the rhubarb filling evenly over the hot crust. Sprinkle the remaining one-third (1/3) of the crumb mixture over the top of 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.

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