Preheat your oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the long sides for easy removal. Lightly grease the parchment paper.

In a large mixing bowl, combine 2 1/4 cups all-purpose flour, 1 cup granulated sugar, and 1/2 teaspoon salt. Whisk briefly to combine.

Add the cold, cubed butter to the dry ingredients. Use a pastry blender, your fingertips, or a food processor to cut the butter into the flour mixture until coarse, pea-sized crumbs form. Be careful not to overmix, as overmixing can lead to a tough crust.

Press about 2/3 of the crumb mixture evenly into the bottom of the prepared baking pan to form the crust. Gently compact it with your hands or the bottom of a flat glass. Reserve the remaining 1/3 of the mixture for the topping.

Bake the crust for 15 minutes, or until lightly golden brown around the edges. Remove from oven and let cool slightly on a wire rack while you prepare the filling.

In a medium saucepan, combine the fresh cranberries, 3/4 cup granulated sugar, 1/4 cup orange juice, 1 teaspoon orange zest, and 1/4 teaspoon ground cinnamon (if using).

Cook over medium heat, stirring occasionally, until the cranberries begin to burst and soften, about 5-7 minutes. Once they start bursting, mash some of them gently with the back of a spoon to release more juice.

In a small bowl, whisk together 2 tablespoons of cornstarch with 1 tablespoon of water to create a smooth slurry. Stir the cornstarch slurry into the simmering cranberry mixture in the saucepan.

Continue to cook, stirring constantly, for another 1-2 minutes, until the filling thickens considerably. It should coat the back of a spoon. Remove from heat.

Pour the warm cranberry filling evenly over the pre-baked crust in the pan. Spread it gently with a rubber spatula to the edges.

Evenly sprinkle the reserved 1/3 of the shortbread crumb mixture over the cranberry filling.

Return the pan to the preheated oven and bake for an additional 25-30 minutes, or until the crumb topping is golden brown and the filling is visibly bubbly around the edges.

Remove the bars from the oven and let them cool completely in the pan on a wire rack before attempting to cut them. This is crucial for the bars to set properly and prevent crumbling. This may take 2-3 hours.

Once the bars are completely cooled, prepare the glaze (if desired). In a small bowl, whisk together 1 cup sifted powdered sugar and 2-3 tablespoons orange juice until smooth and pourable. The consistency should be thick enough to drizzle but not too thin.

Drizzle the glaze evenly over the cooled bars. Let the glaze set for about 15-20 minutes before carefully lifting the bars out of the pan using the parchment paper overhang, then slicing and serving.