Preheat the oven to 325°F (163°C) and grease a 9x13-inch baking pan.

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 mixture into the bottom of the prepared pan and bake for about 15 minutes until just set but not browned.

In the same bowl, beat 4 large eggs until frothy. Add in 2 cups of granulated sugar, ½ cup of all-purpose flour, 1 teaspoon of salt, and optional ½ teaspoon of ground cinnamon. Mix until well combined.

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

Pour the rhubarb filling over the pre-baked crust and spread evenly. Bake for an additional 60 minutes until the center is set.

Cool completely in the pan before slicing into squares and serving.