Put cream and sugar in a large saucepan. Heat on medium to medium-high. Stir until it comes to a boil, then add cream of tartar. Using a candy thermometer, cook and stir constantly till it reaches 238 degrees F. Immediately pour onto a large, cooled marble candy-making slab (or something similar that will stay cold) until the mixture is cooled.
Then mix well until it turns white and mounds. Cover with a damp cloth for 1/2 hour or so.
Then knead it like bread until it's smooth. You can have it plain or add small bits of candied fruit and nuts, if you like, and knead into it.
Roll it into a log on some waxed paper or plastic wrap, and keep refrigerated. Serve by slicing 1/4 inch slices off and serving on a pretty little plate.