Preheat the oven to 325°F (160°C) and prepare a 9x13-inch baking pan by greasing it.

In a large mixing bowl, combine 2 cups of all-purpose flour and ½ cup of powdered sugar. Cut in 1 cup of cold, cubed butter until the mixture resembles coarse crumbs.

Press the crust mixture into the prepared baking pan and bake for 15 minutes or until lightly golden.

In the same bowl, lightly whisk together 4 large eggs until frothy. Gradually stir in 2 cups of granulated sugar, ½ cup of flour, 1 teaspoon of salt, and optionally, ½ teaspoon of ground cinnamon.

Gently fold in 4 cups of diced rhubarb into the filling mixture.

Pour the rhubarb filling over the hot crust. Spread it evenly and bake for an additional 60 minutes, until the filling is set.

Allow the bars to cool completely before slicing into squares or rectangles.