I use this recipe from Martha Stewart, and substitute citric acid for cream of tartar:
Use the lowest possible heat on your stove and it becomes extremely hard to stir once the ingredients set. I use a bit more (just a few drops) extra oil if it is really sticky.
Citric acid (citromsav) is available from the pharmacy and also among spices in large supermarkets.
I make half the quantity given in the recipe, and it works great, keeps fresh for weeks in an airtight container, but dries hard in two days if you leave it out.
Zsuzsi