Preheat your oven to 350°F (175°C) and prepare a baking dish.
In a mixing bowl, cream together the softened butter and sugar until light and fluffy.
Add the egg and vanilla extract to the creamed mixture, mixing until fully combined.
In a separate bowl, whisk together the flour, salt, and cornstarch.
Gradually add the dry mixture into the wet ingredients, mixing just until combined.
Fold in the chopped rhubarb gently into the batter.
Spread the batter evenly into the prepared baking dish, smoothing the top.
For the rhubarb swirl, combine the remaining rhubarb and cornstarch in a small bowl, then spoon over the batter.
Use a knife or skewer to gently swirl the rhubarb into the batter without fully mixing it in.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean and the edges are slightly golden.
Allow the rhubarb bars to cool completely in the dish before slicing into squares and serving.
