Place the molasses, corn syrup, brown sugar, white sugar, vinegar, cream of tartar and half the butter in a pot.
Bring to a boil over low heat until the mixture registers 140° C (260° F). If you don' t have a candy thermometer, don't panic. You'll just have to test it several times.
Stir for 5 to 10 minutes. The mixture has to reach the "ball" stage, meaning it' s ready when you drop a little of it into a small bowl of cold water and it forms a ball.
Mix in the baking soda.
Pour into buttered dishes and let cool slightly until you can pick the taffy up without burning your hands.
Butter your hands well and begin pulling: pull, fold in half, and repeat the process until the taffy is pale golden, and almost white. If it sticks to your hands, put a little more butter on them.
Pull one last time and twist up tightly in small lengths. Cut into pieces with scissors.
Place on a buttered plate or wrap in waxed paper