2 cups unfiltered apple cider
1/2 cup packed light brown sugar
2 tablespoons apple cider vinegar
1 cinnamon stick
Pinch kosher salt
2 tablespoons unsalted butter
Combine the apple cider, sugar, vinegar, cinnamon, and salt in a medium saucepan and bring to a boil over medium-high heat, stirring to dissolve the sugar and salt. Continue simmering until the liquid has thickened slightly and is reduced to about 3/4 cup, 25 to 30 minutes. Remove the pan from the heat and stir in the butter until melted and combined. Cool completely before using.