Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.

Prepare the crust: In a medium bowl, combine the flour, sugar, and salt for the crust. Cut in the cold, cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Press the mixture firmly and evenly into the bottom of the prepared pan.

Bake the crust for 15 minutes until lightly golden brown. While the crust bakes, prepare the filling.

Prepare the filling: In a separate bowl, gently toss the chopped rhubarb with the 3/4 cup of sugar and the 1/4 cup of flour until the rhubarb is evenly coated. Set aside.

In another bowl, whisk together the eggs, heavy cream, and vanilla extract until just combined. Do not overmix.

Pour the egg mixture over the partially baked crust. Gently scatter the sugared rhubarb mixture evenly over the top of the custard layer.

Return the pan to the oven and bake for an additional 40-45 minutes, or until the custard is set (a slight jiggle in the very center is acceptable) and the edges are lightly browned. A toothpick inserted into the custard should come out mostly clean.

Let the bars cool completely in the pan on a wire rack (this is crucial for clean slicing). Once cooled to room temperature, chill in the refrigerator for at least 2 hours before lifting out using the parchment paper overhang and cutting into 12 squares.