Preheat oven to 350°F (175°C). Grease and line an 8x8-inch baking pan with parchment paper.
In a bowl, mix flour, softened butter, and sugar until crumbly. Press evenly into the prepared pan.
Bake the crust for 15–18 minutes until lightly golden.
While the crust bakes, whisk together eggs, sugar, flour, heavy cream, and vanilla extract until smooth. Fold in the diced rhubarb.
Pour the rhubarb custard mixture over the warm crust.
Bake for 35–40 minutes until the center is set and lightly golden.
Remove from oven and sprinkle the top evenly with granulated sugar.
Place under the broiler for 1–2 minutes until the sugar caramelizes into a golden topping. Watch carefully to avoid burning.
Let cool completely before slicing into bars.
