Step 1: Preheat your oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper.
Step 2: Prepare the shortbread dough. In a large mixing bowl, combine the 3 cups of flour, ¾ cup granulated sugar, and ¼ teaspoon salt. Add the cold, cubed butter. Using a pastry blender, two forks, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 3: Press the crust. Reserve about 1 ½ cups of the shortbread mixture for the crumble topping. Press the remaining shortbread mixture evenly into the bottom of the prepared baking pan. Use the bottom of a flat glass or your hands to compact it firmly. Bake the crust for 15-18 minutes, or until lightly golden brown around the edges.
Step 4: While the crust bakes, prepare the rhubarb filling. In a separate large bowl, combine the chopped rhubarb, 1 cup granulated sugar, ¼ cup flour, 1 teaspoon vanilla extract, and optional pinch of cinnamon. Toss gently until the rhubarb is evenly coated.
Step 5: Assemble the bars. Once the crust is lightly golden, carefully remove it from the oven. Spread the rhubarb filling evenly over the warm crust. Crumble the reserved 1 ½ cups of shortbread mixture over the rhubarb layer.
Step 6: Bake the rhubarb shortbread bars. Return the pan to the oven and bake for an additional 35-40 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender when poked with a fork. If the top is browning too quickly, you can lightly tent it with aluminum foil.
Step 7: Cool completely. Let the bars cool completely in the pan on a wire rack before cutting. This can take at least 2-3 hours. Once cooled, use the parchment paper overhang to lift the entire slab out of the pan onto a cutting board. Cut into squares or bars.
Step 8: Storage. Store leftover rhubarb shortbread bars in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to a week. They can also be frozen for up to 3 months.