Step 1 Pour the whiskey into a small sauté pan, and place the pan over medium-high heat. Gently and carefully tilt the pan over the flame to flambé the alcohol (the alcohol will catch fire). Immediately remove from heat and let the flame continue to burn; the flame will self-extinguish when the alcohol is burned out of the whiskey. Set the whiskey aside.
Step 2 In a large saucepan, combine the sugar, water and corn syrup, stirring until the sugar has the consistency of wet sand. Place the saucepan over high heat and cook until the sugar dissolves and begins to boil. Do not stir the sugar, as this may cause it to seize.
Step 3While the sugar is cooking, combine the cream, butter and salt in a saucepan over medium heat. Keep an eye on the sugar while you're heating the cream to keep it from scorching. Cook until the butter melts, stirring it into the cream. When the mixture has come to a simmer, remove from heat.
Step 4Continue to cook the sugar until it darkens to a rich caramel color, 7 to 10 minutes — the sugar will darken quickly and noticeably and will smell faintly nutty. Swirl the pan as the sugar darkens to judge the true color of the caramel (the sugar may darken in patches if there are hot spots on the stove). Watch carefully, as the sugar can easily overcook at this point and burn.
Step 5As soon as the color is a rich caramel, remove the pan from the heat and quickly add the cream mixture in a slow, steady stream. The sugar will bubble and steam as the cream is added; be careful as both the mixture and steam are very hot. Carefully stir in the whiskey and vanilla.
Step 6Stir the caramel until it stops bubbling, then remove to a heat-proof container until needed. This makes about 2½ cups sauce.