Rhubarb Custard Bars

Rhubarb, which looks like raspberry celery, has a short season, so stock up while you can. They freeze perfectly; just store the stems in a sturdy zippered plastic bag. For these custard bars, you can use fresh or frozen rhubarb. In fact, we preferred frozen rhubarb without defrosting.
Preheat the oven to 350 degrees.To make the cake, lightly pour 1 1/2 cups flour into dry measuring cups; flatten with a knife. Combine 1 1/2 cups flour, 1/2 cup sugar, and salt in a bowl. Beat the butter using a pastry blender or 2 knives until the mixture resembles coarse flour. Transfer the mixture to a 13-inch x 9-inch baking dish greased with cooking spray. Bake at 350 ° C for 15 minutes or until the crust is golden brown.To make the filling, lightly pour 1/3 cup flour into a dry measuring cup; smooth with a knife. Combine 1/3 cup flour and 1 1/2 cups sugar in a large bowl; add the milk and eggs, beating with a whisk until smooth. Add the rhubarb. Pour the rhubarb mixture over the cake. Bake at 350 ° C for 40 minutes or until cooked through. Cool to room temperature.To make the filling, put 1/2 cup sugar, cheese and vanilla in a bowl; beat with a mixer on medium speed until smooth. Gently roll in the whipped topping; spread evenly over the baked custard. Cover and refrigerate for at least 1 hour. Garnish with mint sprigs if desired.