Preheat the oven to 350°F (175°C) and grease a 9 x 13-inch baking pan.

In a mixing bowl, combine cold butter, gluten-free flour, and sugar until it resembles coarse crumbs. Press into the prepared pan.

Bake the crust for 10 minutes until lightly golden at the edges.

Chop the rhubarb into small pieces and aim for about 2-3 cups.

In a separate bowl, mix chopped rhubarb, sugar, gluten-free flour, eggs, and heavy cream until smooth.

Pour the custard mixture over the warm crust, spreading evenly.

Bake for an additional 50 to 60 minutes until the center is set.

Let the bars cool to room temperature, then refrigerate for a few hours.

Blend softened cream cheese, sugar, and vanilla until smooth, then fold in whipped cream.

Spread the topping over the cooled custard bars and chill for at least 2 hours.