Preheat oven to 350°F and line a 9x13" pan with parchment paper.
In a large bowl, combine the flour and sugar. Cut in the butter with a fork until crumbly.
Press the crust into the pan and bake for 10 minutes.
While the crust is baking, make the filling.
In a large bowl, whisk together the sugar, flour, half and half, eggs and vanilla; stir in the rhubarb.
Pour the filling into the crust and bake an additional 40-50 minutes or until the filling is set.
Let the bars cool completely.
To make the frosting, whisk together the softened cream cheese, powdered sugar and vanilla; fold in the whipped topping and spread over the cooled bars.
Keep refrigerated until ready to serve.