Put sugar, cream, cream of tartar and corn syrup to a large saucepan and mix well.
Use a pastry brush and a little water to brush all of mixture down the sides of the pan to remove any sugar stuck to the sides.
Cover pan and bring to a boil.
When boiling, uncover and clip a candy thermometer to the side of the pan.
Bring mixture to 240F degrees
(While you are waiting for this to come to temp, butter the bottom and sides of a shallow glass or ceramic dish.)
Remove from stove and add butter and vanilla- do not stir!
Pour mixture into buttered dish (don't use plastic!) and allow this to cool completely (we put it on the back porch where it was cool and this took 2 hours)
Spoon cooled mixture onto clean countertop (it will be like a very sticky caramel)
Work with hands until firm and creamy.
Place this ball of filling in a dish, cover with a damp towel and refrigerate for 2 days.
Bring cream filling to room temp and roll into balls.
Dip into melted chocolate and cool.
You can mix nuts or coconut into the cream if you like, or just leave them plain.