Boil the apple cider, stirring occasionally, in a heavy-bottomed sauce pan over medium-high heat until reduced to 1/4 cup, about 10-15 minutes. Keep an eye on the cider so it doesn't boil over.
Once the apple cider has reduced turn the temperature to low, add the cream, cinnamon, nutmeg and salt to the saucepan. Warm, stirring occasionally, over low heat while preparing the sugar syrup mixture.
Pour 1 cup of water in another heavy-bottomed saucepan. CAREFULLY pour the sugar into the center of the pot taking care to keep the granules away from the sides of the saucepan. Don't stir the mixture. Immediately cover with a lid and bring the mixture to a boil over high heat.
Uncover and insert a candy thermometer and continue to boil until the mixture is thick, light golden brown in color, and registers 300 degrees. This will take about 10-15 minutes.
Reduce the heat to medium and continue cooking until the sugar is a deep amber in color, almost begins to smoke and registers 350 degrees. Remove from heat.
Carefully add 1/4 of the warm cream mixture to the sugar mixture. The mixture will spit & bubble. Allow the mixture to settle down then add the remaining cream mixture; whisk until well combined and smooth. Add the butter and whisk until melted and combined.
Transfer the caramel sauce to a heat-safe container, allow to cool until just warm and serve. The caramel sauce can be stored covered and refrigerated for up to 1 month.
Reheat in the microwave at 50% power. Enjoy!