Put sugar, evaporated milk, evaporated goat's milk, corn syrup and salt in a large heavy pot.
Attach a candy thermometer to the pot.
Cook on medium-low heat for 1 hour, stirring often with a wooden spoon.
The temperature should rise slowly but steadily.
Increase heat to medium-high and cook, stirring constantly, for 20 minutes. Increase heat to high and cook, again, stirring constantly-make sure to scrape the burned bits off the bottom as they develop-until the the temperature reaches 245 degrees.
Immediately remove pot from heat, stir in vanilla and butter and continue to stir mixture occasionally as it cools.
When the candy cools to 180 degrees, stir in pecans (optional) and transfer to a buttered 9 x 12 baking pan.
Cool slightly, cutting into bars while still pliable.