Lightly grease an 8x8-inch baking pan and line it with parchment paper, allowing for an overhang on two sides for easy removal.

In a small bowl, whisk together the cornstarch and ½ cup of orange juice until a smooth slurry forms. Set aside.

In a medium saucepan, combine the remaining ½ cup of orange juice, water, 1 ¼ cups of granulated sugar, orange zest, and salt. Stir over medium heat until the sugar completely dissolves and the mixture begins to simmer.

Slowly pour the cornstarch slurry into the hot orange mixture in the saucepan, whisking constantly to prevent lumps.

Continue cooking over medium-low heat, stirring constantly with a rubber spatula, until the mixture becomes very thick and glossy. This typically takes 15-20 minutes.

Remove the saucepan from the heat and stir in the unsalted butter until it is completely melted and incorporated.

Pour the candy mixture into the prepared 8x8-inch pan and smooth the top with a spatula. Allow it to cool completely to room temperature, then refrigerate for at least 2 hours, or until very firm.

Once firm, spread the remaining ½ cup of granulated sugar in a shallow dish. Using the parchment paper overhang, lift the candy block from the pan and place it on a cutting board. Cut the candy into desired shapes, such as 1-inch squares or rectangles.

Roll each piece of candy in the granulated sugar to thoroughly coat all sides. Place the coated candies on a clean tray.

Allow the candies to air-dry at room temperature for several hours (or overnight) to develop a slight crust.

Store the orange candies in an airtight container at room temperature. If stacking, place parchment paper between layers to prevent sticking.