Whisk together milk and sweetener in a medium saucepan over medium-low heat. Whisking often, bring to a low simmer. Continue to simmer, whisking often, until milk has reduced by half, about 30 minutes. Once reduced, remove from heat and stir in butter and vanilla. Allow to cool completely and store in a mason jar in the refrigerator for up to 1 week.
Makes 1 1/4 cups.Nutrition calculations are based on using whole dairy milk and granulated sugar. To make sugar-free: Use your favorite heat-safe sugar substitute in the appropriate amounts for the recipe.