Preheat oven to 350°F (175°C) and grease a 9x13-inch pan.
In a bowl, mix softened butter, powdered sugar, vanilla, and flour to form a crumbly dough.
Press dough evenly into the bottom of the pan to form the crust.
In a large bowl, whisk together eggs, sugar, flour, and salt until smooth.
Gently fold in chopped rhubarb to the custard mixture.
Pour the filling over the prepared crust.
Bake for 45–50 minutes or until the top is golden and the custard is set.
Cool completely before slicing and serving.