Preheat your oven to 350°F (175°C) and grease a 9x13-inch pan. In a mixing bowl, combine all-purpose flour, granulated sugar, and salt. Cut in cold, cubed unsalted butter until the mixture resembles coarse crumbs. Press into the bottom of the prepared pan and bake for 15 minutes or until lightly golden.

In a separate bowl, whisk together the large eggs, granulated sugar, flour, salt, and pure vanilla extract until smooth and well combined. Gently fold in the chopped fresh rhubarb.

Remove the crust from the oven and pour the rhubarb custard mixture over the warm crust, spreading it evenly. Bake for 40-45 minutes until the top is golden and the center jiggles slightly.

Let the bars cool to room temperature, then refrigerate for 1-2 hours. Dust with powdered sugar if desired, slice into squares, and serve.