Step 1: Preheat your oven to 350°F (175°C) and line an 8x8 inch square baking pan with parchment paper.
Step 2: In a large bowl, combine 2 cups of flour, 1/2 cup sugar, and salt. Cut in the cold butter using a pastry cutter or two knives until the mixture resembles coarse crumbs.
Step 3: Reserve 1 cup of the crumb mixture for the topping. Press the remaining mixture firmly into the bottom of the prepared pan.
Step 4: Bake the crust for 12-15 minutes or until it is just beginning to turn a light golden color.
Step 5: While the crust bakes, toss the chopped rhubarb with 3/4 cup sugar, cornstarch, vanilla extract, and lemon zest in a separate bowl.
Step 6: Spread the rhubarb mixture evenly over the pre-baked crust, then sprinkle the reserved 1 cup of crumbs over the top.
Step 7: Return to the oven and bake for an additional 30-35 minutes, or until the rhubarb is bubbling and the top is golden brown. Cool completely before cutting into bars.