1. Grease a 9-inch-by-13-inch pan and line it with plastic wrap. Greasing the bottom and sides helps the plastic stick to the pan. Set aside until needed.
2. To make the sugar syrup, combine the sugar, cream of tartar and water in a large saucepan. Cook over high heat, stirring constantly, until the sugar is dissolved, 5 to 10 minutes. Wash the sides of the pan with a pastry brush dipped in hot water and clip a candy thermometer to the side of the pan. Continue cooking over medium-high heat without stirring until the mixture reaches 290 F. Remove from the heat and stir until the temperature drops a few degrees to prevent scorching. The temperature will first rise a few more degrees and then fall. Cover the saucepan.
3. To make the cornstarch paste, whisk together the cornstarch, lemon juice and water in a large saucepan. Cook over medium heat, whisking constantly, until the mixture turns into a thick paste, 5 to 10 minutes. Pour the sugar syrup into the cornstarch paste in five or six batches, whisking after each batch until smooth. Raise the temperature to medium-high and continue cooking, stirring constantly with a wooden spoon to prevent scorching, until the mixture turns a light golden color and is very thick and gelatinous, about 30 minutes.
4. Remove the pan from the heat, add the rose water or orange blossom water, and stir until smooth.
5. Scrape the mixture into the prepared pan and spread with wooden spoon. The mixture will be hard to spread, so don’t worry if it’s imperfect. Cover with plastic wrap and cool completely at room temperature, 5 hours or overnight.
6. When the candy is completely cool, remove the slab from the pan and cut into 1-inch pieces (make seven cuts along the width and 11 cuts along the length). Sift together the confectioner’s sugar and cornstarch in a large mixing bowl and whisk to combine. Toss several pieces of the candy at a time into the confectioners’ sugar-cornstarch mixture to coat. Store in an airtight container between layers of plastic wrap or parchment paper.
— Dinah Bucholz, "The Unofficial Narnia Cookbook"
Cooking tip: I substituted 1 teaspoon vanilla for the rosewater or orange blossom water. Also, the batch I tried cooked faster than the times she listed, so keep an eye on it as it’s cooking. The Turkish Delight absorbed the confectioner’s sugar-cornstarch coating, so you may need to re-toss them right before serving.