Preheat the oven to 350°F (175°C).
Prepare an 8x8 inch baking dish by greasing or lining it with parchment paper.
In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the flour and salt, mixing until just combined.
Press the dough evenly into the bottom of the prepared baking dish.
Bake the crust in the preheated oven for about 15 minutes, or until it is lightly golden.
While the crust is baking, prepare the rhubarb filling by combining the chopped rhubarb, brown sugar, cornstarch, vanilla, and egg in a saucepan.
Cook the mixture over medium heat until the rhubarb has softened and the sugar has dissolved, about 5-7 minutes.
Pour the rhubarb filling over the pre-baked crust and swirl it gently with a knife.
Return the dish to the oven and bake for an additional 25-30 minutes, or until the filling is set and slightly bubbling.
Remove from the oven and let it cool completely before slicing into bars.
