In a large saucepan with a candy thermometer, combine corn syrup, water, and sugar. Heat on medium and stir often. Sugar will dissolve. Continue heating and stirring until temperature reaches 250 F.
Once you reach 250, carefully add butter chunks and stir, then carefully add heavy cream mixture while stirring. The temperature will drop by about 30 degrees.
Continue to cook at medium heat, stirring very frequently as the temperature continues to climb. The mixture will bubble and darken slowly. As soon as the thermometer reaches 245 F remove from heat.
Stir in 1 teaspoon of course salt. (* if using vanilla extract, stir it in here as well)
Pour the mixture into the lined pan
After about 1 minute, as soon as bubbles appear to have settled out, liberally sprinkle the 1-2 teaspoons of finishing/fancy salt across the entire surface of the caramel.
Allow to sit undisturbed on the counter overnight, at least 12 hours.