This caramel is not as soft as ice cream topping, and works well as a layer in bars and pies, as it firms up better in the refrigerator. You will need a candy thermometer. Calibrate it before you begin so you can adjust the temperature to your altitude.
In a very heavy saucepan or cast iron pan, spread the sugar in an even layer. Slowly cook the sugar over low heat. Watch it very carefully, slightly pushing the sugar around with a heat-proof spoon or spatula as it liquefies and starts to darken in color. Don’t over stir or you might make the sugar crystallize.
When all the sugar is melted, (still being careful not to let the sugar burn) take the pan off the heat and add half the cream. It will bubble up quite a bit. Stir caramel until it begins to mix together (there will be a lump of firm sugar that needs to melt into the cream. Just give it time.)
Add the remaining cream and let sauce cook down until all the bits of sugar dissolve.
Return to heat and cook, stirring constantly until mixture reaches 230 degrees F. (sea-level temp. calibrate your thermometer accordingly)
Add the butter, salt, and vanilla. Stir in well.
Put the sauce through a sieve to catch any little hard bits of sugar.