Preheat your oven to 350°F. In a bowl, combine the flour, sugar, and salt. Add the cold butter cubes and work them into the flour using your fingertips or a pastry cutter until the mixture resembles coarse breadcrumbs—this keeps the crust tender and flaky. Press this mixture firmly into the bottom of a 9x13 inch baking dish, creating an even layer. Bake for 20 minutes until the crust is light golden and set. I like to use King Arthur all-purpose flour here because it creates a more reliable, tender texture that won't turn tough.

While the crust bakes, prepare the filling. Add the lemon juice, 1 cup of the sliced rhubarb, and the eggs to a bowl. Using an immersion blender, blend until smooth and well combined—this creates a silky custard base and naturally incorporates the rhubarb juice for flavor and moisture. The blender also ensures the eggs are fully emulsified, which helps the filling set properly.

To the lemon-rhubarb mixture from Step 2, stir in the lemon zest, sugar, and flour until just combined—this thickens the filling slightly and ensures even baking. Gently fold in the remaining 1 cup of raw rhubarb slices and the vanilla extract. For the best texture, I keep some rhubarb pieces whole rather than blending all of them, so you get pockets of tart fruit throughout the bars. Pour this filling over the warm crust from Step 1, spreading it evenly.

Bake at 350°F for 35 minutes, until the filling is set but still slightly jiggles in the very center—this means it will have a tender, custardy texture rather than becoming rubbery. The edges should look set and slightly golden. Let the bars cool completely in the pan at room temperature (about 1 hour), then refrigerate for at least 2 hours before cutting into squares. Chilling helps them set properly and makes them easier to cut cleanly.