Pour the milk into a medium non-stick pot. Heat over medium heat until it reaches a gentle simmer. Do not let it boil.
Lower the heat to medium-low and let the milk simmer gently for about 50 minutes, stirring occasionally every 5–10 minutes with a spatula to prevent the bottom from browning. The milk will reduce slightly—about ¼—and remain runny.
Whisk in the sweetener until completely dissolved. Continue simmering for about 1 hour, or until the milk has reduced by half, taken on a creamy color, and thickened slightly. Do not stir foam or residue around the pot sides into the milk.
Remove the pot from heat immediately. Strain the condensed milk into a heatproof container and allow it to cool completely before sealing with an airtight lid.
Label and store in the refrigerator. The milk will thicken further as it cools and can last up to 3-4 weeks in the fridge.