How to Make It
Using a long-handled wooden or heatproof metal spoon, stir together sugar and water in a deep, heavy 10-inch skillet or medium-size enameled cast-iron saucepan over medium. Cook, stirring occasionally, until sugar is mostly dissolved. Increase heat to medium-high, and bring mixture to a boil, stirring constantly. Cook, stirring, until mixture turns into a clear syrup. Continue to cook, stirring constantly, until mixture dries out, turning sandy and crumbly, and then begins to melt again. Cook, stirring often, until melted mixture darkens to an amber color, 8 to 10 minutes. Remove pan from heat.
Carefully pour in heavy cream, stirring constantly. (Caramel will bubble up, boil vigorously, and steam profusely.) Stir constantly until boiling gradually subsides. Return pan to medium-high; cook, stirring until mixture is smooth. Remove pan from heat; stir in vanilla extract, kosher salt, and, if desired, apple brandy. Let Caramel Sauce stand until room temperature, 15 to 20 minutes.
Transfer room-temperature Caramel Sauce to a jar or container with an airtight lid. Keep refrigerated up to 2 weeks. Warm over low heat before using.