Preheat your oven to 350°F (175°C). In a large mixing bowl, combine 2 ½ cups of all-purpose flour, 1 cup of granulated sugar, and a pinch of salt. Cut in 1 cup of cold, cubed unsalted butter until the mixture resembles coarse crumbs. Press this crumbly dough into a greased 9x13 inch baking pan. Bake for 15 minutes until the crust is lightly golden at the edges.

In a medium bowl, whisk together 4 large eggs, 1 ¼ cups of granulated sugar, ½ teaspoon of vanilla extract, and 2 tablespoons of all-purpose flour until smooth and well combined. Gently fold in 2 cups of chopped fresh rhubarb, ensuring every piece is coated in the custard mixture.

Once the crust is out of the oven, carefully pour the creamy rhubarb custard over the warm crust, spreading it evenly. Bake the Rhubarb Custard Bars in the oven for 40–45 minutes, until the edges are set and the center is slightly jiggly but not liquid.

After baking, remove the pan from the oven and let it cool on a wire rack for at least 1 hour. For the cleanest slices, refrigerate the bars for an additional hour before cutting. Dust with powdered sugar before serving.