Combine the molasses, corn syrup, and vinegar in a saucepan, and cook, stirring occasionally to 240°F.
Then continue cooking, stirring constantly, to 270°F or until a little of the mixture dropped into cold water is slightly brittle.
When done, add the butter and stir only enough to mix.
Sprinkle the salt over the corn in a large bowl.
Then slowly pour the syrup over it, while mixing well.
Then quickly shape into balls, using as little pressure as possible.