Add water, sugar, and salt, and vanilla bean, if using, to a medium saucepan and heat over medium to medium-high heat, stirring well to dissolve the sugar.
Let the mixture simmer for 10-15 minutes, watching it to make sure it doesn't boil too hard, or burn. You are watching for the mixture to turn golden brown. Darker brown yields a more robust flavor. Be patient, it takes several minutes!
As soon as the mixture turns golden brown, stir in the heavy cream and vanilla, using a long handled wood spoon or heat resistant spatula. Be careful as the mixture will bubble and foam up rapidly. You need to keep stirring!
Keep cooking mixture until the candy thermometer reads 225°F. The hotter temperature you cook it to, the firmer it will end up, but for a sauce, this is the right temperature.
To cool mixture quicker, pour into a bowl and set in a bigger bowl filled with ice water and stir to cool. Make sure the water isn't too deep. You don't want water in your lovely caramel sauce!
When cooled, pour into a jar and put the lid on it. Serve over your favorite treats!