Preheat your oven to 350°F (175°C) and prepare a 9×13-inch baking pan with parchment paper or grease lightly with butter.

Combine all-purpose flour and sugar in a mixing bowl. Cut in the butter until the mixture resembles coarse crumbs. Press into the bottom of the prepared pan and bake for 10 minutes.

In another bowl, whisk together granulated sugar, flour, half-and-half, eggs, and vanilla until smooth. Fold in the diced rhubarb.

Pour the custard filling over the hot crust and return to the oven. Bake for 40 to 50 minutes or until the custard is set.

Once baked, cool the bars to room temperature, then refrigerate for at least 1 hour.

For the optional topping, beat together cream cheese, powdered sugar, and vanilla until smooth, and fold in whipped topping. Spread over the cooled bars.

Return the bars to the refrigerator and chill until set, then cut into squares and serve.