Start by preheating your oven to 350°F or 175°C. Line your baking pan with parchment paper for easy removal. Leave some paper hanging on the sides. This helps lift the bars out later without breaking them. Lightly grease the paper for extra safety.

In a bowl, mix flour, powdered sugar and salt. Add cold butter cubes and rub with fingers. Keep mixing until it looks like soft crumbs. Press this mixture firmly into the pan base. Bake for 15 minutes until slightly golden and set.

Wash and chop rhubarb into small pieces. Set aside in a bowl. In another bowl, whisk eggs and sugar until smooth. Add flour, lemon juice, lemon zest and vanilla extract. Mix gently until everything becomes creamy and well combined.

Now fold chopped rhubarb into the lemon mixture. Stir slowly so rhubarb spreads evenly. The filling should look slightly thick and creamy. This step is important because it ensures every bite has balanced sweet and tart flavors throughout the bars.

Pour the filling over the warm crust. Spread it evenly with a spatula. Place the pan back into the oven. Bake for 25 to 30 minutes until the center is set. The top should look lightly golden and slightly firm when touched.

Remove the pan from the oven and let it cool completely. This step is very important for clean slices. Once cooled, lift the bars using parchment paper. Cut into squares or rectangles. Dust with powdered sugar for a soft finishing touch before serving.