Preheat oven to 350°F (175°C).
In a large bowl, mix 2 cups flour and 1/3 cup sugar.
Cut in cold butter until the mixture resembles coarse crumbs.
Press into a greased 9x13-inch baking dish.
Bake for 10–12 minutes until lightly golden.
Meanwhile, whisk sugar, flour, milk, eggs, and vanilla until smooth.
Fold in diced rhubarb.
Pour mixture over the warm crust.
Bake an additional 40–50 minutes, until custard is set but slightly jiggly in center.
Cool completely in the pan on a wire rack.
(Optional) For topping: Beat cream cheese, powdered sugar, and vanilla until smooth. Fold in whipped topping. Spread over cooled bars.
Chill at least 1 hour before cutting into squares.