Preheat oven to 350°F (175°C). Lightly grease and line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal.

Prepare the crust: In a medium bowl, cream together the softened butter and 1/2 cup sugar until light and fluffy. Gradually mix in 1 cup of flour and salt until just combined. Press the mixture evenly into the bottom of the prepared baking pan.

Par-bake the crust for 15 minutes. While the crust bakes, prepare the filling. In a separate bowl, toss the chopped rhubarb with 1/2 cup of sugar and 1/4 cup of flour until coated.

Prepare the custard: In a large bowl, whisk together the remaining 1 cup of sugar, eggs, milk, and vanilla extract until smooth. Fold in the coated rhubarb mixture.

Pour the rhubarb custard mixture evenly over the hot par-baked crust. Return the pan to the oven and bake for an additional 30-35 minutes, or until the custard is set and lightly golden brown around the edges.

Let the bars cool completely in the pan on a wire rack (this is crucial for setting). Once cool, use the parchment overhang to lift the entire slab out of the pan before slicing into 16 squares.