Cook over medium heat, stirring occasionally, until the sugar has dissolved.
Reduce the heat to low and bring the milk to a simmer. Once the milk starts to simmer, do not stir it to prevent it from crystalizing.
Continue simmering the milk for 35 to 45 minutes, until the milk begins to thicken and turns a darker, creamy color. If foam begins to form on top, lightly scrape it off with a spoon.
When the milk is finished reducing, remove it from heat and allow it to cool completely.
Transfer the cooled milk to a jar or other airtight container and keep refrigerated until ready to use.