Melt the sugar, butter, corn syrup and salt in a large pot. Once it comes to a boil, slowly drizzle in the evaporated milk, stirring constantly. Do this slowly enough that it takes about 15 minutes PER CAN. To get a time reference, I used a 1 ounce measuring glass and counted to 60 as I poured. It is truly just a drizzle so that the milk doesn’t curdle and separate. After all the milk is added, bring the mixture to soft ball stage (238 degrees F) using a candy thermometer. 238 degrees will produce a soft caramel, 240 degrees will be slightly chewy and anything more than that will have the hard, sucker-like consistency of a Sugar Daddy. All of them are good, it just depends on what you want. Once at temperature, immediately remove from heat and stir in the Vanilla extract. Pour into a 9 x 13 parchment lined pan and allow to cool before cutting. Wrap individual pieces in wax paper and keep in an airtight jar. Refrigerating them will make them last longer, but they will be harder the colder they are.