In a heavy-bottomed saucepan, combine white sugar, citric acid, and water over medium heat. Stir continuously until the sugar dissolves completely, then increase the heat to bring the mixture to a boil. Once boiling, reduce the heat and simmer until the syrup reaches 250°F (hard ball stage), approximately 10-15 minutes.

While the syrup is cooking, mix cornstarch with water in a separate bowl until smooth. This mixture will serve as the thickening agent. Set it aside to allow any bubbles to settle. Prepare your workspace by greasing a container for setting.

Once the syrup has reached the desired temperature, gradually whisk in the cornstarch mixture, ensuring it blends well without lumps. Return the mixture to medium-low heat and continue stirring continuously to prevent sticking.

Keep the mixture simmering on low heat, stirring every 6-7 minutes to prevent scorching. Continue this for 1 to 2 hours, until a medium-dark amber color is achieved.

Once thickened, remove the mixture from heat. Stir in the measured rosewater and, if desired, red food coloring. Pour the mixture into the prepared container and let it sit at room temperature for at least 5 hours or overnight.

After fully setting, dust the top with cornstarch. Remove the mixture from the container and cut into 1-inch cubes, dredging each piece in a mixture of cornstarch and powdered sugar.