Add the apple cider to a medium heavy-bottomed saucepan. Simmer over medium heat, stirring occasionally, until reduced to about ½ cup, 12 to 18 minutes.

Stir in the brown sugar and butter until dissolved and melted. Keep at a gentle simmer over medium-low so the bubbles stay small and steady.

Reduce heat to low, then slowly pour in the heavy cream while whisking. The mixture will bubble. Simmer 4 to 6 minutes, stirring often, until glossy and slightly thick.

Remove from heat. Whisk in the cinnamon and vanilla. Taste, then finish with a pinch of flaky salt or save it for garnish.

Let the caramel cool until slightly thickened before storing. It will continue to thicken as it cools.