1. Combine the sugar, corn syrup, and water in a medium saucepan. If using a vanilla bean, scrape the seeds and add the seeds and pod to the pan. Bring to a boil over medium-high heat. You can stir the mixture before it boils to help dissolve the sugar, but stop stirring the moment it starts. Boil the syrup until it’s a medium amber color. Once it starts to color, tilt the pan occasionally—it’s easier to see the true color of the caramel when you’re looking at less of it (or it may seem darker than it really is). As soon as it’s close to medium amber, turn off the heat—the caramel will retain some heat, so it’s good to allow for carryover cooking.
2. Add the butter to the caramel and stir gently to combine. Stir in the cream (be careful—the caramel may bubble up and steam a lot! Just keep stirring and it will die down). The sauce should be smooth and creamy. If it seizes up and you see chunks of caramel that haven’t fully dissolved in the sauce, return the pan to low heat and heat, stirring occasionally, until the sauce is smooth. Stir in the salt (and vanilla, if using). Remove from the heat and let cool to room temperature.