Combine sugar, 1-1/2 cup water and lemon juice in a heavy saucepan. Bring to a boil, stirring constantly. Boil vigorously for 5 to 10 minutes, until the syrup reaches 240°F on a candy thermometer, or forms a soft ball when a bit is dropped into cold water. Turn off the heat.

Using a blender, food processor, or whisk, combine the cornstarch and cream of tartar, then gradually add 3 cups of water, stirring vigorously to fully combine the ingredients and prevent lumping. Transfer this mixture to a large saucepan, and bring to a boil over medium heat, stirring constantly. If any lumps form, scoop them out of the pan; don't try to break them up to make them smooth. It won't work.

Once the cornstarch mixture has come to a boil, pour in the hot syrup in a thin, steady stream, stirring constantly. Reduce the heat and simmer the mixture for 1 to 2 hours, stirring constantly, until the mixture has turned a pale gold. Turn off the heat. Stir in flavoring to taste, and food coloring, if desired. Blend in the nuts, if desired.

Using a flavorless oil, lightly oil a 9" square baking pan, then line the pan with lightly oiled baker's parchment. Pour the lokoum into the pan, then tilt it to distribute the mixture evenly. Wait until the lokoum has cooled completely before covering the pan with plastic wrap. Do not allow the plastic to touch the surface of the lokoum, or it will stick mercilessly.

Let the lokoum rest for at least two days before cutting into 1" wide strips with an oiled kitchen knife (not serrated). Clean and oil the knife after every cut. If the lokoum is too gummy to cut, let it age longer. Lay out the strips on a lightly oiled tray and let them rest for another day or two before cutting into small squares.

Combine one-half cup each of cornstarch and powdered sugar in a tightly covered container. Put 2 or 3 squares of lokoum into the container, cover and shake to coat them with the mixture. Store in an airtight container, separating the layers with parchment, waxed paper or doilies.