Preheat your oven to 350°F (175°C). Grease and line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.

Prepare the crust: In a medium bowl, combine the 1 cup flour, 1/4 cup sugar, and salt. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs.

Press the crust mixture firmly and evenly into the bottom of the prepared baking pan. Bake the crust for 15 minutes until lightly golden brown. Remove from the oven and set aside.

Prepare the filling: In a separate bowl, whisk together the 1 cup sugar and 1/3 cup flour. Add the chopped rhubarb, egg, vanilla extract, and almond extract (if using). Gently toss everything together until the rhubarb is evenly coated.

Pour the rhubarb filling evenly over the warm, pre-baked crust. Return the pan to the oven and bake for an additional 30-35 minutes, or until the filling is bubbly and set, and the edges are lightly browned.

Allow the bars to cool completely in the pan on a wire rack. Cooling thoroughly (at least 2 hours, or chilling is recommended) is crucial for clean cutting. Once cool, lift the parchment paper overhang to remove the slab from the pan.

Cut into 12 bars and dust generously with powdered sugar before serving.