Preheat your oven to 350°F (175°C).
Prepare an 8x8-inch baking dish by greasing it lightly.
In a mixing bowl, combine flour, 1/4 cup sugar, and salt.
Add softened butter and mix until crumbly.
Press the crumb mixture firmly into the bottom of the prepared baking dish.
Bake the crust for 10 minutes until lightly golden.
In a separate bowl, mix chopped rhubarb, brown sugar, cornstarch, vanilla, and egg.
Spread the rhubarb filling over the pre-baked crust.
Using a knife, swirl the filling to create a marbled effect.
Return the dish to the oven and bake for an additional 25-30 minutes.
Allow the bars to cool completely in the pan before slicing.
Dust with powdered sugar before serving, if desired.