Bring the water and sugar to a boil. Combine the cream of tartar with one tablespoon cold water, add to the boiling water and stir. Remove the foam from the surface.
Blend the corn starch with ¼ cup water. Add and cook over low heat for 15-20 minutes, until the syrup thickens.
Add the pistachios to the syrup and stir.
Pour the prepared Turkish delight syrup into a deep, square oven dish and let it cool. After cooled, bake in a 320°F (160°C) preheated oven for 10 minutes. Remove from oven and let cool.
After the Turkish delight cools completely, cut into cubes, sprinkle powdered sugar on top and serve.
(Two pieces of lokum equals one serving.)