Preheat your oven to 350°F. Grease a 9x13-inch baking dish or line it with parchment paper.
In a mixing bowl, combine the flour, softened butter, powdered sugar, and salt. Mix until it resembles coarse crumbs.
Press the mixture firmly into the bottom of the prepared baking dish to form the crust.
In another bowl, toss the chopped rhubarb with granulated sugar, cornstarch, lemon juice, and vanilla extract until well combined. Spread the rhubarb mixture evenly over the crust.
In a separate bowl, whisk together the egg and baking powder. Pour this mixture over the rhubarb layer, spreading it gently to cover.
Bake in the preheated oven for 35-40 minutes, or until the top is set and lightly golden.
Allow the bars to cool completely in the pan before cutting into squares.
For best results, chill the bars in the refrigerator for at least 1 hour before serving.