To a small saucepan over medium-high heat, add the apple cider and cinnamon. Cook for 18-20 minutes until it is reduced to about 0.25 cup. Set aside.

Add the granulated sugar to a separate medium heavy-bottomed saucepan. Spread it out evenly over the bottom of the pan. Set the saucepan over medium-low heat. Do not stir the sugar at this point. Allow the sugar to begin melting on its own.

As the sugar starts to melt around the edges, gently tilt the pan to help the sugar melt evenly. Once you see a significant amount of melted sugar, you can start gently stirring with a wooden spoon or heatproof silicone spatula to ensure all the sugar melts evenly. (Be patient. It may seem slow at first, but the sugar will eventually melt completely and begin to turn golden.)

Continue to cook the sugar, stirring gently, until it reaches a deep amber color (about 5-10 minutes). Be careful not to let it burn.

Once the sugar is a deep amber color, reduce the heat to low. Carefully add the butter, stirring constantly until melted.

Gradually add the warm heavy cream while stirring. The mixture will bubble up, so be cautious.

Stir in the reduced apple cider and vanilla until well combined. Add a pinch of kosher salt and stir to incorporate.

Continue cooking on low until smooth, about 5-10 minutes.

Remove from heat and allow the sauce to thicken as it cools. Use warm or cold.