Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

For the crust: In a large bowl, cream together the softened butter, granulated sugar, and light brown sugar until light and fluffy. Beat in the vanilla extract. Gradually add the flour and mix until just combined, forming a crumbly dough.

Press the dough evenly into the bottom of the prepared baking pan to form the crust. Bake for 15 minutes.

While the crust bakes, prepare the rhubarb filling: In a medium saucepan, combine the chopped rhubarb, granulated sugar, cornstarch, and water. Cook over medium heat, stirring constantly, until the rhubarb has softened and the mixture has thickened, about 5-7 minutes.

Remove the partially baked crust from the oven and spread the rhubarb filling evenly over the top.

For the topping: In a small bowl, mix together the melted butter, light brown sugar, and flour until well combined. Stir in the chopped nuts, if using. Crumble this mixture evenly over the rhubarb layer.

Return the pan to the oven and bake for another 30-35 minutes, or until the topping is golden brown and the filling is bubbly.

Let the bars cool completely in the pan on a wire rack before cutting into squares. This is crucial for them to set properly.