Reduce milk. Add milk to a saucepan over medium heat. Bring to a simmer before reducing to low and simmering for 20-30 minutes. Mix occasionally.
You will know it's done when it has reduced by about half and develops a rich beige color.
Store. Strain through cheesecloth or a mesh strainer to remove any solids. Cool and store in an airtight jar in the fridge for up to 5 days.