Prepare the Baking Dish:Line an 8x8-inch baking dish with parchment paper and lightly dust it with a mixture of powdered sugar and cornstarch.
Make the Sugar Syrup:In a medium saucepan, combine sugar, citric acid (or lemon juice), and 1 1/2 cups of water. Heat over medium heat, stirring until the sugar dissolves. Attach a candy thermometer and let the syrup reach 240°F (soft-ball stage).
Prepare the Starch Mixture:In another saucepan, whisk 1 cup of cornstarch and cream of tartar with the remaining 2 1/2 cups of water. Cook over medium heat, whisking constantly, until the mixture thickens and resembles a smooth paste
Combine and Cook:Gradually pour the hot sugar syrup into the starch mixture, whisking continuously. Lower the heat and cook for 30-40 minutes, stirring frequently, until the mixture turns golden and jelly-like.
Add Flavor and Color:Remove the mixture from heat. Stir in rosewater and food coloring (if using) until evenly combined.
Set the Lokum:Pour the mixture into the prepared baking dish, spreading it evenly. Let it cool at room temperature, then cover and refrigerate for 4-6 hours, or until firm.
Cut and Coat:Dust a clean surface with the powdered sugar and cornstarch mixture. Invert the Lokum onto the surface, peel off the parchment paper, and cut into bite-sized cubes. Toss the cubes in the sugar-cornstarch coating to prevent sticking.
Serve and Enjoy:Store in an airtight container at room temperature for up to two weeks.